Different via configurations for different via interface requirements

ABSTRACT

Vias, along with methods for fabricating vias, are disclosed that exhibit reduced capacitance and resistance. An exemplary interconnect structure includes a first source/drain contact and a second source/drain contact disposed in a dielectric layer. The first source/drain contact physically contacts a first source/drain feature and the second source/drain contact physically contacts a second source/drain feature. A first via having a first via layer configuration, a second via having a second via layer configuration, and a third via having a third via layer configuration are disposed in the dielectric layer. The first via and the second via extend into and physically contact the first source/drain contact and the second source/drain contact, respectively. A first thickness of the first via and a second thickness of the second via are the same. The third via physically contacts a gate structure, which is disposed between the first source/drain contact and the second source/drain contact.

The present application is a non-provisional application of and claimsbenefit of U.S. Patent Application Ser. No. 62/907,823, filed Sep. 30,2019, the entire disclosure of which is incorporated herein byreference.

BACKGROUND

The integrated circuit (IC) industry has experienced exponential growth.Technological advances in IC materials and design have producedgenerations of ICs, where each generation has smaller and more complexcircuits than the previous generation. In the course of IC evolution,functional density (i.e., the number of interconnected IC devices perchip area) has generally increased while geometry size (i.e., dimensionsand/or sizes of IC features and/or spacings between these IC features)has decreased. Typically, scaling down has been limited only by anability to lithographically define IC features at the ever-decreasinggeometry sizes. However, resistance-capacitance (RC) delay has arisen asa significant challenge as reduced geometry sizes are implemented toachieve ICs with faster operating speeds (e.g., by reducing distancestraveled by electrical signals), thereby negating some of the advantagesachieved by scaling down and limiting further scaling down of ICs. RCdelay generally indicates delay in electrical signal speed through an ICresulting from a product of resistance (R) (i.e., a material'sopposition to flow of electrical current) and capacitance (C) (i.e., amaterial's ability to store electrical charge). Reducing both resistanceand capacitance is thus desired to reduce RC delay and optimizeperformance of scaled down ICs. Interconnects of ICs, which physicallyand/or electrically connect IC components and/or IC features of the ICs,are particularly problematic in their contributions to RC delay. A needthus exists for improvements in interconnects of ICs and/or methods offabricating the interconnects.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1A and FIG. 1B are a flow chart of a method for fabricating aportion of a multilayer interconnect structure according to variousaspects of the present disclosure.

FIGS. 2-6 , FIGS. 7A-11A, FIGS. 7B-11B, FIGS. 7C-11C, and FIGS. 12-16are fragmentary diagrammatic views of an integrated circuit device, inportion or entirety, at various stages of fabricating a multilayerinterconnect structure, such as the method for fabricating themultilayer interconnect structure of the integrated circuit device ofFIG. 1A and FIG. 1B, according to some embodiments of the presentdisclosure.

FIGS. 17A-17C are fragmentary diagrammatic views of an integratedcircuit device, in portion or entirety, having a multilayer interconnectstructure, which can be fabricated by the method of FIG. 1A and FIG. 1B,according to other embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure relates generally to integrated circuit devices,and more particularly, to multilayer interconnect structures forintegrated circuit devices.

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,spatially relative terms, for example, “lower,” “upper,” “horizontal,”“vertical,” “above,” “over,” “below,” “beneath,” “up,” “down,” “top,”“bottom,” etc. as well as derivatives thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) are used for ease of the presentdisclosure of one features relationship to another feature. Thespatially relative terms are intended to cover different orientations ofthe device including the features. Furthermore, when a number or a rangeof numbers is described with “about,” “approximate,” and the like, theterm is intended to encompass numbers that are within a reasonable rangeconsidering variations that inherently arise during manufacturing asunderstood by one of ordinary skill in the art. For example, the numberor range of numbers encompasses a reasonable range including the numberdescribed, such as within +/−10% of the number described, based on knownmanufacturing tolerances associated with manufacturing a feature havinga characteristic associated with the number. For example, a materiallayer having a thickness of “about 5 nm” can encompass a dimension rangefrom 4.5 nm to 5.5 nm where manufacturing tolerances associated withdepositing the material layer are known to be +/−10% by one of ordinaryskill in the art. Still further, the present disclosure may repeatreference numerals and/or letters in the various examples. Thisrepetition is for the purpose of simplicity and clarity and does not initself dictate a relationship between the various embodiments and/orconfigurations discussed.

As IC technologies expand into sub-20 nm technology nodes, shrinkingcritical dimensions at device layers of ICs (e.g., gate lengths, gatepitches, fin pitches, etc.) have led to corresponding shrinking incritical dimensions of the interconnects (e.g., dimensions of vias,dimensions of metal lines, via pitches, metal line pitches, etc.) thatfacilitate operation of the device layers. This has presented uniquechallenges. For example, typically, vias to source/drain contacts (whichare connected to source/drain features) and vias to gate structures havethe same configuration and are fabricated using the same methods—fillinga via opening in a dielectric layer with a metal fill layer disposedover a metal glue/barrier layer, where the metal glue/barrier layer isconfigured to enhance adhesion between the vias and the dielectric layer(for example, the metal glue/barrier layer lines sidewalls of the viaopening defined by the dielectric layer), reduce contact resistance,and/or prevent diffusion of via constituents into the surroundingenvironment. As via CDs shrink, the metal glue/barrier layer consumesmore space in the via opening, thereby reducing a remainder of the viaopening for the metal fill layer. This has led to poor metal gapfilling, where the metal fill layer is unable to fill the remainder ofthe via opening without forming gaps (or voids), which significantlyincreases contact resistance. One solution is to eliminate the metalglue/barrier layer and fabricate barrier-free vias. However, it has beenobserved that some vias, such as vias to gate structures, need the metalglue/barrier layer to optimize contact resistance reduction, while othervias, such as vias to source/drain contacts, need to eliminate the metalglue/barrier layer to optimize contact resistance reduction.

The present disclosure thus proposes mixing via configurations toaccommodate different via interface requirements. For example, thepresent disclosure proposes fabricating vias at a same level of amulti-level interconnect (MLI) structure (for example, vias at avia-zero (M0), bottommost via layer) with differentconfigurations/structures to optimize contact resistance reduction. Insome embodiments, vias to gate structures include a metal glue/barrierlayer, while vias to source/drain contacts do not include a metalglue/barrier layer. For the vias to the source/drain contacts, theproposed via fabrication methods include forming via openings in adielectric layer that expose the source/drain contacts, recessing thesource/drain contacts to extend the via openings, filling the extendedvia openings with a first metal fill material using a bottom-updeposition process, forming a metal glue/barrier layer over the firstmetal fill material (for example, by a conformal deposition process),forming a second metal fill material over the metal glue/barrier layer(for example, by a blanket deposition process), and performing aplanarization process that removes any second metal fill material, metalglue/barrier layer, and/or first metal fill material disposed over a topsurface of the dielectric layer. For the vias to the gate structures,the proposed via fabrication methods include forming via openings in adielectric layer that expose the gate structures, forming a second metalglue/barrier layer along sidewalls of the via openings defined by thedielectric layer and a bottom of the via openings defined by the gatestructure (for example, by a conformal deposition process), forming athird metal fill material over the second metal glue/barrier layer andfilling a remainder of the via openings (for example, by a blanketdeposition process), and performing a planarization process that removesany second metal glue/barrier layer and/or third metal fill materialdisposed over the top surface of the dielectric layer. In someembodiments, the vias to the gate structures are formed before formingthe vias to the source/drain contacts. In some embodiments, the vias tothe gate structures are formed after forming the vias to thesource/drain contacts.

Recessing the source/drain contacts increases a contact area between thevias and the source/drain contacts. Process variations of the bottom-updeposition process result in different heights of the first metal fillmaterial. For example, the first metal fill material may completely filla first via opening to a first source/drain contact, while the firstmetal fill material may partially fill a second via opening to a secondsource/drain contact. In some embodiments, the first metal fill materialcompletely fills the first via opening and extends above the top surfaceof the dielectric layer to form a via rivet head. As a height of the viarivet head increases (defined between a topmost surface of the via rivethead and the top surface of the dielectric layer), internal stressincreases within the first metal fill material, which can lead to cracksduring continual growth of the first metal fill material and/or duringsubsequent processing. In some embodiments, the bottom-up depositionprocess is tuned to constrain the height of the via rivet head to apredefined height that can minimize internal stress of the first metalfill material. In some embodiments, the metal glue/barrier layer isformed over the first metal fill material before forming the secondmetal fill material (which is needed to fill a remainder of via openingsthat are partially filled by the first metal fill material) to reduceinternal stress in the first metal fill material and prevent cracking ofmetal fill layers of the vias. Such fabrication methods result in somevias to source/drain contacts having barrier-free interfaces and somevias to source/drain contacts having partial barrier interfaces. Detailsof the proposed methods for fabricating vias and resulting viastructures and/or configurations are described herein.

FIG. 1A and FIG. 1B are a flow chart of a method 10 for fabricating aportion of a multilayer interconnect structure of an integrated circuitdevice according to various aspects of the present disclosure. Theportion of the multilayer interconnect structure fabricated by method 10can reduce capacitance and/or resistance associated with the IC device,thereby reducing associated RC delay. At block 20, method 10 includesforming a first source/drain contact and a second source/drain contactin a dielectric layer. The first source/drain contact physicallycontacts a first source/drain feature. The second source/drain contactphysically contacts a second source/drain feature. At block 30, method10 includes forming a first via opening and a second via opening in thedielectric layer. The first via opening exposes the first source/draincontact and the second via opening exposes the second source/draincontact. At block 40, method 10 includes recessing the firstsource/drain contact to extend the first via opening and recessing thesecond source/drain contact to extend the second via opening. At block50, a bottom-up deposition process is performed to form a first via bulklayer in the first via opening and a second via bulk layer in the secondvia opening. In some embodiments, a first thickness of the first viabulk layer is different (for example, greater or less) than a secondthickness of the second via bulk layer. In some embodiments, a firstthickness of the first via bulk layer is the same as a second thicknessof the second via bulk layer. At block 60 and block 70, a first viabarrier layer is formed over the first via bulk layer and the second viabulk layer and a third via bulk layer is formed over the first viabarrier layer, respectively.

At block 80, method 10 includes performing a planarization process toremove any of the third via bulk layer, the first via barrier layer, thesecond via bulk layer, and the first via bulk layer that is disposedover a top surface of the dielectric layer, thereby forming a first viahaving a third thickness and a first via layer configuration and asecond via having the third thickness and a second via layerconfiguration that is different than the first via layer configuration.At block 90, block 100, and block 110, a third via opening is formed inthe dielectric layer that exposes a gate structure, a second via barrierlayer is formed that partially fills the third via opening, and a fourthvia bulk layer is formed over the second via barrier layer,respectively. The fourth via bulk layer fills a remainder of the thirdvia opening. At block 120, method 10 includes performing a planarizationprocess to remove any of the fourth via bulk layer and the second viabarrier layer that is disposed over the top surface of the dielectriclayer, thereby forming a third via having a third via layerconfiguration that is different than the first via layer configurationand the second via layer configuration. In some embodiments, the firstvia, the second via, and the third via are a portion of a bottommost vialayer of the multilayer interconnect structure. In some embodiments,fabrication can proceed with forming additional layers of the multilayerinterconnect structure, such as respective conductive lines over andphysically contacting the first via, the second via, and the third via.Additional processing is contemplated by the present disclosure.Additional steps can be provided before, during, and after method 10,and some of the steps described can be moved, replaced, or eliminatedfor additional embodiments of method 10. The following discussionprovides interconnects that can be fabricated according to method 10.

FIGS. 2-6 , FIGS. 7A-11A, FIGS. 7B-11B, FIGS. 7C-11C, and FIGS. 12-16are fragmentary diagrammatic views of an integrated circuit (IC) device200, in portion or entirety, at various stages of fabricating amultilayer interconnect structure of IC device 200 (such as thoseassociated with method 100 in FIG. 1 ), according to various aspects ofthe present disclosure. FIGS. 2-6 , FIGS. 7A-11A, and FIGS. 12-16 arefragmentary cross-sectional views of IC device 200 in an X-Z plane atvarious fabrication stages according to various aspects of the presentdisclosure. FIGS. 7B-11B and FIGS. 7C-11C are fragmentarycross-sectional views of portions of IC device 200 in FIGS. 7A-11A in aY-Z plane along lines at various fabrication stages according to variousaspects of the present disclosure. IC device 200 may be included in amicroprocessor, a memory, and/or other IC device. In some embodiments,IC device 200 may be a portion of an IC chip, a system on chip (SoC), orportion thereof, that includes various passive and activemicroelectronic devices such as resistors, capacitors, inductors,diodes, p-type FETs (PFETs), n-type FETs (NFETs),metal-oxide-semiconductor FETs (MOSFETs), complementary MOS (CMOS)transistors, bipolar junction transistors (BJTs), laterally diffused MOS(LDMOS) transistors, high voltage transistors, high frequencytransistors, other suitable components, or combinations thereof. Thevarious transistors may be planar transistors or multi-gate transistors,such as FinFETs, depending on design requirements of IC device 200.FIGS. 2-6 , FIGS. 7A-11A, FIGS. 7B-11B, FIGS. 7C-11C, and FIGS. 12-16have been simplified for the sake of clarity to better understand theinventive concepts of the present disclosure. Additional features can beadded in IC device 200, and some of the features described below can bereplaced, modified, or eliminated in other embodiments of IC device 200.

Turning to FIG. 2 , IC device 200 includes a substrate (wafer) 210. Inthe depicted embodiment, substrate 210 includes silicon. Alternativelyor additionally, substrate 210 includes another elementarysemiconductor, such as germanium; a compound semiconductor, such assilicon carbide, gallium arsenide, gallium phosphide, indium phosphide,indium arsenide, and/or indium antimonide; an alloy semiconductor, suchas silicon germanium (SiGe), GaAsP, AlInAs, AlGaAs, GalnAs, GaInP,and/or GaInAsP; or combinations thereof. Alternatively, substrate 210 isa semiconductor-on-insulator substrate, such as a silicon-on-insulator(SOI) substrate, a silicon germanium-on-insulator (SGOI) substrate, or agermanium-on-insulator (GOI) substrate. Semiconductor-on-insulatorsubstrates can be fabricated using separation by implantation of oxygen(SIMOX), wafer bonding, and/or other suitable methods. Substrate 210includes doped regions formed by an ion implantation process, adiffusion process, and/or other suitable doping process. In someembodiments, substrate 210 includes p-type doped regions (for example,p-type wells) doped with p-type dopants, such as boron, indium, otherp-type dopant, or combinations thereof. In some embodiments, substrate210 includes n-type doped regions (for example, n-type wells) doped withn-type dopants, such as phosphorus, arsenic, other n-type dopant, orcombinations thereof. In some embodiments, substrate 210 includes dopedregions formed with a combination of p-type dopants and n-type dopants.The various doped regions can be formed directly on and/or in substrate210, for example, providing a p-well structure, an n-well structure, adual-well structure, a raised structure, or combinations thereof.

Isolation features can be formed over and/or in substrate 210 to isolatevarious regions, such as device regions, of IC device 200. For example,isolation features define and electrically isolate active device regionsand/or passive device regions from each other. Isolation featuresinclude silicon oxide, silicon nitride, silicon oxynitride, othersuitable isolation material (for example, including silicon, oxygen,nitrogen, carbon, or other suitable isolation constituent), orcombinations thereof. Isolation features can include differentstructures, such as shallow trench isolation (STI) structures, deeptrench isolation (DTI) structures, and/or local oxidation of silicon(LOCOS) structures. In some embodiments, isolation features are formedby etching a trench (or trenches) in substrate 210 and filling thetrench with insulator material (for example, using a chemical vapordeposition (CVD) process or a spin-on glass process). A chemicalmechanical polishing (CMP) process may be performed to remove excessiveinsulator material and/or planarize a top surface of isolation features.In some embodiments, isolation features can be formed by depositing aninsulator material over substrate 210 after forming fin structures (insome embodiments, such that the insulator material fills gaps (trenches)between the fin structures) and etching back the insulator material. Insome embodiments, isolation features include a multi-layer structurethat fills trenches, such as a bulk dielectric layer disposed over aliner dielectric layer, where the bulk dielectric layer and the linerdielectric layer include materials depending on design requirements (forexample, a bulk dielectric layer that includes silicon nitride disposedover a liner dielectric layer that includes thermal oxide). In someembodiments, isolation features include a dielectric layer disposed overa doped liner layer (including, for example, boron silicate glass (BSG)or phosphosilicate glass (PSG)).

Various gate structures are disposed over substrate 210, such as a gatestructure 230A, a gate structure 230B, and a gate structure 230C. Eachof gate structures 230A-230C engage a respective channel region definedbetween a respective source region and a respective drain region(referred to hereinafter as source/drain regions), such that current canflow between the respective source/drain regions during operation. Insome embodiments, gate structures 230A-230C are formed over a finstructure, such that gate structure 230A-230C each wrap a portion of thefin structure and interpose respective source/drain regions of the finstructure. Gate structures 230A-230C each include a metal gate (MG)stack 232. MG stacks 232 are formed by deposition processes, lithographyprocesses, etching processes, other suitable processes, or combinationsthereof. The deposition processes include CVD, physical vapor deposition(PVD), atomic layer deposition (ALD), high density plasma CVD (HDPCVD),metal organic CVD (MOCVD), remote plasma CVD (RPCVD), plasma enhancedCVD (PECVD), low-pressure CVD (LPCVD), atomic layer CVD (ALCVD),atmospheric pressure CVD (APCVD), plasma enhanced ALD (PEALD), plating,other suitable methods, or combinations thereof. The lithographypatterning processes include resist coating (for example, spin-oncoating), soft baking, mask aligning, exposure, post-exposure baking,developing the resist, rinsing, drying (for example, hard baking), othersuitable processes, or combinations thereof. Alternatively, thelithography exposure process is assisted, implemented, or replaced byother methods, such as maskless lithography, electron-beam writing, orion-beam writing. The etching processes include dry etching processes,wet etching processes, other etching processes, or combinations thereof.MG stacks 232 are fabricated according to a gate last process, a gatefirst process, or a hybrid gate last/gate first process. In gate lastprocess embodiments, gate structures 230A-230C include dummy gate stacksthat are subsequently, completely or partially, replaced with MG stacks232. The dummy gate stacks include, for example, an interfacial layer(for example, a silicon oxide layer) and a dummy gate electrode layer(for example, a polysilicon layer) disposed over the interfacial layer.In such embodiments, the dummy gate electrode layer is removed, therebyforming openings that are filled with MG stacks 232. In someembodiments, the dummy gate stacks include a dummy gate dielectric layerdisposed between the interfacial layer and the dummy gate electrodelayer, which can be removed during the gate replacement process. In someembodiments, the dummy gate dielectric layer and/or the interfaciallayer are not removed during the gate replacement process and form aportion of MG stacks 232.

MG stacks 232 are configured to achieve desired functionality accordingto design requirements of IC device 200, such that MG stacks 232 of gatestructures 230A-230C may include the same or different layers and/ormaterials relative to one another. In some embodiments, MG stacks 232include a gate dielectric (for example, a gate dielectric layer) and agate electrode (for example, a work function layer and a bulk conductivelayer). MG stacks 232 may include numerous other layers, for example,capping layers, interface layers, diffusion layers, barrier layers, hardmask layers, or combinations thereof. In some embodiments, the gatedielectric layer is disposed over an interfacial layer (including adielectric material, such as silicon oxide), and the gate electrode isdisposed over the gate dielectric layer. The gate dielectric layerincludes a dielectric material, such as silicon oxide, high-k dielectricmaterial, other suitable dielectric material, or combinations thereof.Examples of high-k dielectric material include hafnium dioxide (HfO₂),HfSiO, HfSiON, HfTaO, HfTiO, HfZrO, zirconium oxide, aluminum oxide,hafnium dioxide-alumina (HfO₂—Al₂O₃) alloy, other suitable high-kdielectric materials, or combinations thereof. High-k dielectricmaterial generally refers to dielectric materials having a highdielectric constant (k value) relative to a dielectric constant ofsilicon dioxide (k≈3.9). For example, high-k dielectric material has adielectric constant greater than about 3.9. In some embodiments, thegate dielectric layer is a high-k dielectric layer. The gate electrodeincludes a conductive material, such as polysilicon, Al, Cu, Ti, Ta, W,Mo, Co, TaN, NiSi, CoSi, TiN, WN, TiAl, TiAlN, TaCN, TaC, TaSiN, otherconductive material, or combinations thereof. In some embodiments, thework function layer is a conductive layer tuned to have a desired workfunction (such as an n-type work function or a p-type work function),and the conductive bulk layer is a metal layer formed over the workfunction layer. In some embodiments, the work function layer includesn-type work function materials, such as Ti, Ag, Mn, Zr, TaAl, TaAlC,TiAlN, TaC, TaCN, TaSiN, other suitable n-type work function materials,or combinations thereof. In some embodiments, the work function layerincludes a p-type work function material, such as Ru, Mo, Al, TiN, TaN,WN, ZrSi₂, MoSi₂, TaSi₂, NiSi₂, WN, other suitable p-type work functionmaterials, or combinations thereof. The bulk (or fill) conductive layerincludes a suitable conductive material, such as Al, W, and/or Cu. Thebulk conductive layer may additionally or collectively includepolysilicon, Ti, Ta, metal alloys, other suitable materials, orcombinations thereof.

Gate structures 230A-230C further include gate spacers 236, which aredisposed adjacent to (for example, along sidewalls of) MG stacks 232.Gate spacers 236 are formed by any suitable process and include adielectric material. The dielectric material can include silicon,oxygen, carbon, nitrogen, other suitable material, or combinationsthereof (for example, silicon oxide, silicon nitride, siliconoxynitride, or silicon carbide). For example, in the depictedembodiment, a dielectric layer including silicon and nitrogen, such as asilicon nitride layer, can be deposited over substrate 210 andsubsequently anisotropically etched to form gate spacers 236. In someembodiments, gate spacers 236 include a multi-layer structure, such as afirst dielectric layer that includes silicon nitride and a seconddielectric layer that includes silicon oxide. In some embodiments, morethan one set of spacers, such as seal spacers, offset spacers,sacrificial spacers, dummy spacers, and/or main spacers, are formedadjacent to MG stacks 232. In such embodiments, the various sets ofspacers can include materials having different etch rates. For example,a first dielectric layer including silicon and oxygen (for example,silicon oxide) can be deposited over substrate 210 and etched to form afirst spacer set adjacent to MG stacks 232, and a second dielectriclayer including silicon and nitrogen (for example, silicon nitride) canbe deposited over substrate 210 and etched to form a second spacer setadjacent to the first spacer set. Implantation, diffusion, and/orannealing processes may be performed to form lightly doped source anddrain (LDD) features and/or heavily doped source and drain (HDD)features in substrate 210 before and/or after forming gate spacers 236.

Epitaxial source features and epitaxial drain features (referred to asepitaxial source/drain features), such as an epitaxial source/drainfeature 240A, an epitaxial source/drain feature 240B, an epitaxialsource/drain feature 240C, and an epitaxial source/drain feature 240D,are disposed in source/drain regions of substrate 210. Gate structure230A interposes epitaxial source/drain feature 240A and epitaxialsource/drain feature 240B, such that a channel region is defined betweenepitaxial source/drain feature 240A and epitaxial source/drain feature240B. Gate structure 230B interposes epitaxial source/drain feature 240Band epitaxial source/drain feature 240C, such that a channel region isdefined between epitaxial source/drain feature 240B and epitaxialsource/drain feature 240C. Gate structure 230C interposes epitaxialsource/drain feature 240C and epitaxial source/drain feature 240D, suchthat a channel region is defined between epitaxial source/drain feature240C and epitaxial source/drain feature 240D. In some embodiments, gatestructure 230A, epitaxial source/drain feature 240A, and epitaxialsource/drain feature 240B form a portion of a first transistor; gatestructure 230B, epitaxial source/drain feature 240B, and epitaxialsource/drain feature 240C form a portion of a second transistor; andgate structure 230C, epitaxial source/drain feature 240C, and epitaxialsource/drain feature 240D form a portion of a third transistor.

In some embodiments, a semiconductor material is epitaxially grown onand/or from substrate 210 to form epitaxial source/drain features240A-240D over source/drain regions of substrate 210. In someembodiments, an etching process is performed on source/drain regions ofsubstrate 210 to form source/drain recesses, where epitaxialsource/drain features 240A-240D are grown to fill the source/drainrecesses. In some embodiments, where substrate 210 represents a portionof a fin structure, epitaxial source/drain features 240A-240D wrapsource/drain regions of the fin structure and/or are disposed insource/drain recesses of the fin structure. An epitaxy process canimplement CVD deposition techniques (for example, vapor-phase epitaxy(VPE), ultra-high vacuum CVD (UHV-CVD), LPCVD, and/or PECVD), molecularbeam epitaxy, other suitable SEG processes, or combinations thereof. Theepitaxy process can use gaseous and/or liquid precursors, which interactwith the composition of substrate 210. Epitaxial source/drain features240A-240D are doped with n-type dopants and/or p-type dopants. In someembodiments, epitaxial source/drain features 240A-240D are epitaxiallayers including silicon and/or carbon, where the silicon-comprisingepitaxial layers or the silicon-carbon-comprising epitaxial layers aredoped with phosphorous, other n-type dopant, or combinations thereof. Insome embodiments, epitaxial source/drain features 240A-240D areepitaxial layers including silicon and germanium, where thesilicon-and-germanium-compromising epitaxial layers are doped withboron, other p-type dopant, or combinations thereof. In someembodiments, epitaxial source/drain features 240A-240D include materialsand/or dopants that achieve desired tensile stress and/or compressivestress in the channel regions. In some embodiments, epitaxialsource/drain features 240A-240D are doped during deposition by addingimpurities to a source material of the epitaxy process. In someembodiments, epitaxial source/drain features 240A-240D are doped by anion implantation process subsequent to a deposition process. In someembodiments, annealing processes are performed to activate dopants inepitaxial source/drain features 240A-240D and/or other source/drainfeatures (for example, HDD regions and/or LDD regions).

A multilayer interconnect (MLI) feature 250 is disposed over substrate210. MLI feature 250 electrically couples various devices (for example,transistors, resistors, capacitors, and/or inductors) and/or components(for example, gate structures 230A-230C and/or source/drain features240A-240D) of IC device 200, such that the various devices and/orcomponents can operate as specified by design requirements of IC device200. MLI feature 250 includes a combination of dielectric layers andconductive layers (for example, metal layers) configured to form variousinterconnects. The conductive layers are configured to form verticalinterconnects, such as device-level contacts and/or vias, and/orhorizontal interconnects, such as conductive lines. Verticalinterconnects typically connect horizontal interconnects in differentlayers (or different planes) of MLI feature 250. During operation of ICdevice 200, the interconnects route signals between the devices and/orthe components of IC device 200 and/or distribute signals (for example,clock signals, voltage signals, and/or ground signals) to the devicesand/or the components of IC device 200, along with routing anddistributing signals between the devices and/or components of IC device200 with devices and/or components external to IC device 200. MLIfeature 250 is depicted with a given number of dielectric layers andconductive layers. The present disclosure contemplates MLI feature 250having more or less dielectric layers and/or conductive layers dependingon design requirements.

MLI feature 250 includes one or more insulating layers disposed oversubstrate 210, such as an interlayer dielectric (ILD) layer 252 (ILD-0),an interlayer dielectric (ILD) layer 254 (ILD-1), a contact etch stoplayer (CESL) 262, and a contact etch stop layer (CESL) 264. ILD layer252 is disposed over substrate 210, and ILD layer 254 is disposed overILD layer 252. CESL 262 is disposed between ILD layer 252 and substrate210, epitaxial source/drain features 240A-240C, and/or gate structures230A-230C (in particular, gate spacers 236). CESL 264 is disposedbetween ILD layer 252, ILD layer 254, and/or gate structures 230A-230C.In some embodiments, a thickness of ILD layer 252 is about 10 nm toabout 25 nm, a thickness of ILD layer 254 is about 10 nm to about 25 nm,a thickness of CESL 262 is about 1 nm to about 10 nm, and a thickness ofCESL 264 is about 1 nm to about 10 nm. ILD layers 252, 254 and/or CESLs262, 264 are formed over substrate 210 by a deposition process, such asCVD, PVD, ALD, HDPCVD, MOCVD, RPCVD, PECVD, LPCVD, ALCVD, APCVD, PEALD,other suitable methods, or combinations thereof. In some embodiments,ILD layer 252 and/or ILD layer 254 is formed by a high aspect ratioprocess (HARP), such as HDPCVD, which generally refers to a depositionprocess having parameters configured to achieve adequate filling in highaspect ratio structures. In some embodiments, ILD layer 252 and/or ILDlayer 254 is formed by a flowable CVD (FCVD) process that includes, forexample, depositing a flowable material (such as a liquid compound) oversubstrate 210 and converting the flowable material to a solid materialby a suitable technique, such as thermal annealing and/or treating theflowable material with ultraviolet radiation. Subsequent to thedeposition of ILD layer 252, ILD layer 254, CESL 262, and/or CESL 264, aCMP process and/or other planarization process may be performed, suchthat ILD layers 252, 254 and/or CESLs 262, 264 have substantially planarsurfaces.

ILD layers 252, 254 include a dielectric material including, forexample, silicon oxide, carbon doped silicon oxide, silicon nitride,silicon oxynitride, tetraethyl orthosilicate (TEOS), PSG, BSG,boron-doped phosphosilicate glass (BPSG), fluorine-doped silicate glass(FSG), Black Diamond® (Applied Materials of Santa Clara, Calif.),xerogel, aerogel, amorphous fluorinated carbon, parylene,benzocyclobutene (BCB)-based dielectric material, SiLK (Dow Chemical,Midland, Mich.), polyimide, other suitable dielectric material, orcombinations thereof. In some embodiments, ILD layers 252, 254 include alow-k dielectric material, which generally refers to a dielectricmaterial having a low dielectric constant relative to the dielectricconstant of silicon dioxide (k≈3.9). For example, low-k dielectricmaterial has a dielectric constant less than about 3.9. In someembodiments, the low-k dielectric material has a dielectric constantless than about 2.5, which can be referred to as an extreme low-k (ELK)dielectric material. In the depicted embodiment, ILD layers 252, 254include an ELK dielectric material (and thus can be referred to as anELK dielectric layer), such as silicon dioxide (SiO₂) (for example,porous silicon dioxide), silicon carbide (SiC), and/or carbon-dopedoxide (for example, a SiCOH-based material (having, for example, Si—CH₃bonds)), each of which is tuned/configured to exhibit a dielectricconstant less than about 2.5. CESLs 262, 264 include a materialdifferent than ILD layers 252, 254, such as a dielectric material thatis different than the dielectric material of ILD layer 252, 254. Forexample, where ILD layers 252, 254 include silicon and oxygen (forexample, SiCOH, SiO_(x), or other silicon-and-oxygen comprising materialhaving a dielectric constant less than about 2.5), CESLs 262, 264 caninclude silicon and nitrogen and/or carbon (for example, SiN, SiCN,SiCON, SiON, SiC, and/or SiCO) (and can thus be referred to as a siliconnitride layer). In some embodiments, CESLs 262, 264 include a metaloxide and/or a metal nitride layer. ILD layer 252, 254 and/or CESLs 262,264 can include a multilayer structure having multiple dielectricmaterials.

Turning to FIG. 3 , one or more interconnect openings, such as aninterconnect opening 270A and an interconnect opening 270B, are formedin a dielectric layer by a patterning process. Interconnect opening 270Aand interconnect opening 270B extend vertically through ILD layer 254,CESL 264, ILD layer 252, and CESL 262 to expose epitaxial source/drainfeature 240B and epitaxial source/drain feature 240C, respectively.Interconnect openings 270A, 270B may thus be referred to as source/draincontact (plug) openings. Interconnect opening 270A includes a sidewall272A (defined by ILD layer 254, CESL 264, ILD layer 252, and CESL 262),a sidewall 274A (defined by ILD layer 254, CESL 264, ILD layer 252, andCESL 262), and a bottom 276A (defined by epitaxial source/drain feature240B) that extends between sidewall 272A and sidewall 274A. Interconnectopening 270B includes a sidewall 272B (defined by ILD layer 254, CESL264, ILD layer 252, and CESL 262), a sidewall 274B (defined by ILD layer254, CESL 264, ILD layer 252, and CESL 262), and a bottom 276B (definedby epitaxial source/drain feature 240C) that extends between sidewall272B and sidewall 274B. In FIG. 3 , each of interconnect openings 270A,270B has a trapezoidal shape, though the present disclosure contemplatesinterconnect openings 270A, 270B having other shapes, such as arectangular shape. Sidewalls 272A, 274A are tapered, such that a bottomwidth of interconnect opening 270A exposing epitaxial source/drainfeature 240B is less than a top width of interconnect opening 270A at atop surface of ILD layer 254. A width of interconnect opening 270A thusdecreases along the z-direction from the top width of interconnectopening 270A to the bottom width of interconnect opening 270A. Sidewalls272B, 274B are tapered, such that a bottom width of interconnect opening270B exposing epitaxial source/drain feature 240C is less than a topwidth of interconnect opening 270B at a top surface of ILD layer 254. Awidth of interconnect opening 270B thus decreases along the z-directionfrom the top width of interconnect opening 270B to the bottom width ofinterconnect opening 270B.

In some embodiments, the patterning process includes performing alithography process to form a patterned mask layer 278 having an opening279A (substantially aligned with epitaxial source/drain feature 240B)and an opening 279B (substantially aligned with epitaxial source/drainfeature 240C) therein over ILD layer 254 and performing an etchingprocess to transfer a pattern defined in patterned mask layer 278 to anunderlying dielectric layer (here, ILD layer 254, CESL 264, ILD layer252, and CESL 262). The lithography process can include forming a resistlayer on ILD layer 254 (for example, by spin coating), performing apre-exposure baking process, performing an exposure process using amask, performing a post-exposure baking process, and performing adeveloping process. During the exposure process, the resist layer isexposed to radiation energy (such as ultraviolet (UV) light, deep UV(DUV) light, or extreme UV (EUV) light), where the mask blocks,transmits, and/or reflects radiation to the resist layer depending on amask pattern of the mask and/or mask type (for example, binary mask,phase shift mask, or EUV mask), such that an image is projected onto theresist layer that corresponds with the mask pattern. Since the resistlayer is sensitive to radiation energy, exposed portions of the resistlayer chemically change, and exposed (or non-exposed) portions of theresist layer are dissolved during the developing process depending oncharacteristics of the resist layer and characteristics of a developingsolution used in the developing process. After development, thepatterned resist layer includes a resist pattern that corresponds withthe mask. Alternatively, the exposure process can be implemented orreplaced by other methods, such as maskless lithography, electron-beamwriting, and/or ion-beam writing.

In some embodiments, the patterned resist layer is patterned mask layer278. In such embodiments, the patterned resist layer is used as an etchmask to remove portions of the underlying dielectric layer (here, ILDlayer 254, CESL 264, ILD layer 252, and/or CESL 262) exposed by openings279A, 279B. In some embodiments, the patterned resist layer is formedover a mask layer formed over the underlying dielectric layer beforeforming the resist layer, and the patterned resist layer is used as anetch mask to remove portions of the mask layer formed over theunderlying dielectric layer, thereby forming patterned mask layer 278.In such embodiments, the patterned mask layer is used as an etch mask toremove portions of ILD layer 254, CESL 264, ILD layer 252, and CESL 262exposed by openings 279A, 279B. The etching process can include a dryetching process (for example, a reactive ion etching (RIE) process), awet etching process, other suitable etching process, or combinationsthereof. Various selective etching processes can be performed to forminterconnect openings 270A, 270B. For example, the etching process caninclude a first etch that selectively etches ILD layer 254 relative topatterned mask layer 278 and CESL 264, such that the first etch stopsupon reaching CESL 264; a second etch that selectively etches CESL 264relative to ILD layers 254, 252, such that the second etch stops uponreaching ILD layer 252; a third etch that selectively etches ILD layer252 relative to CESLs 262, 264, such that the third etch stops uponreaching CESL 262; and a fourth etch that selectively etches CESL 262relative to ILD layers 252, 254 and epitaxial source/drain features240B, 240C, such that the fourth etch stops upon reaching epitaxialsource/drain features 240B, 240C. In some embodiments, the first etch,the second etch, the third etch, and the fourth etch may be configuredto slightly over etch. For example, the first etch may partially etchCESL 264, the second etch may partially etch ILD layer 252, the thirdetch may partially etch CESL 262, and/or the fourth etch may partiallyetch epitaxial source/drain features 240B, 240C. In some embodiments,the first etch, the second etch, the third etch, and the fourth etch aresimilar to etch processes described herein for etching ILDs and CESLs.In some embodiments, the etching process may include multiple steps foretching CESLs 262, 264. In some embodiments, the etching processimplements an etchant with low etch selectivity between ILD layers 254,252 and CESLs 262, 264, such that interconnect openings 270A, 270B areformed by in a single etch step based on, for example, time. In someembodiments, after the etching process, patterned mask layer 278 isremoved from ILD layer 254 (in some embodiments, by a resist strippingprocess). In some embodiments, patterned mask layer 278 is removedduring etching of ILD layer 254, CESL 264, ILD layer 252, and/or CESL262.

Turning to FIG. 4 , silicide layers 280 are formed on epitaxialsource/drain features 240B, 240C. Silicide layers 280 extend throughCESL 262. In the depicted embodiment, top surfaces of silicide layers280 are disposed higher than a top surface of CESL 262 relative to a topsurface of substrate 210. In some embodiments, the top surfaces ofsilicide layers 280 are disposed lower and/or substantially planar withthe top surface of CESL 262 relative to the top surface of substrate210. In some embodiments, the top surfaces of silicide layers 280 aredisposed lower than the top surface of substrate 210. Silicide layers280 may be formed by depositing a metal layer over epitaxialsource/drain features 240B, 240C and heating IC device 200 (for example,subjecting IC device 200 to an annealing process) to cause constituentsof epitaxial source/drain features 240B, 240C (for example, siliconand/or germanium) to react with metal constituents of the metal layer.The metal layer includes any metal constituent suitable for promotingsilicide formation, such as nickel, platinum, palladium, vanadium,titanium, cobalt, tantalum, ytterbium, zirconium, other suitable metal,or combinations thereof. Silicide layers 280 thus include a metalconstituent and a constituent of epitaxial source/drain features 240B,240C, such as silicon and/or germanium. In some embodiments, silicidelayers 280 include nickel silicide, titanium silicide, or cobaltsilicide. Any un-reacted metal, such as remaining portions of the metallayer, may be selectively removed relative to silicide layers 280 and/ora dielectric material, for example, by an etching process.

Then, a source/drain contact 282A and a source/drain contact 282B arerespectively formed in interconnect opening 270A and interconnectopening 270B. Source/drain contacts 282A, 282B each extend through ILDlayer 254, CESL 264, and ILD layer 252 to silicide layers 280, such thatsource/drain contacts 282A, 282B are disposed on silicide layers 280,respectively, on epitaxial source/drain features 240B, 240C. In someembodiments, depending on a configuration of silicide layers 280,source/drain contacts 282A, 282B can extend partially or entirelythrough CESL 262. Source/drain contacts 282A, 282B each include acontact barrier layer 284 and a contact bulk layer 286 disposed overcontact barrier layer 284. Contact barrier layer 284 is disposed onsidewalls 272A, 272B, sidewalls 274A, 274B, and bottoms 276A, 276Brespectively of interconnect openings 270A, 270B. For example, contactbarrier layer 284 physically contacts sidewalls 272A, 272B, sidewalls274A, 274B, and bottoms 276A, 276B respectively of interconnect openings270A, 270B. In some embodiments, source/drain contacts 282A, 282B areformed by performing a first deposition process to form a contactbarrier material over ILD layer 254 that partially fills interconnectopenings 270A, 270B and performing a second deposition process to form acontact bulk material over the contact barrier material, where thecontact bulk material fills a remainder of interconnect openings 270A,270B. In such embodiments, contact barrier material and contact bulkmaterial are disposed in interconnect openings 270A, 270B and over thetop surface of ILD layer 254. The first deposition process and thesecond deposition process can include CVD, PVD, ALD, HDPCVD, MOCVD,RPCVD, PECVD, LPCVD, ALCVD, APCVD, PEALD, electroplating, electrolessplating, other suitable deposition methods, or combinations thereof. Insome embodiments, contact barrier layer 284 has a substantially uniformthickness along sidewalls 272A, 272B, sidewalls 274A, 274B, and bottoms276A, 276B respectively of interconnect openings 270A, 270B. Contactbarrier layer 284 may thus be formed by a conformal deposition process.A CMP process and/or other planarization process is performed to removeexcess contact bulk material and contact barrier material, for example,from over the top surface of ILD layer 254, resulting in source/draincontacts 282A, 282B (in other words, contact barrier layer 284 andcontact bulk layer 286 filling interconnect openings 270A, 270B). TheCMP process planarizes a top surface of source/drain contacts 282A,282B, such that in some embodiments, the top surface of ILD layer 254and top surfaces of source/drain contacts 282A, 282B form asubstantially planar surface.

Contact barrier layer 284 includes a material that promotes adhesionbetween a surrounding dielectric material (here, ILD layer 254, CESL264, and/or ILD layer 252) and contact bulk layer 286. The material ofcontact barrier layer 284 may further prevent diffusion of metalconstituents (for example, metal atoms/ions) form source/drain contacts282A, 282B into the surrounding dielectric material. In someembodiments, contact barrier layer 284 includes titanium, titaniumalloy, tantalum, tantalum alloy, cobalt, cobalt alloy, ruthenium,ruthenium alloy, molybdenum, molybdenum alloy, palladium, palladiumalloy, other suitable constituent configured to promote and/or enhanceadhesion between a metal material and a dielectric material and/orprevent diffusion of metal constituents from the metal material to thedielectric material, or combinations thereof. For example, contactbarrier layer 284 includes tantalum, tantalum nitride, tantalum aluminumnitride, tantalum silicon nitride, tantalum carbide, titanium, titaniumnitride, titanium silicon nitride, titanium aluminum nitride, titaniumcarbide, tungsten, tungsten nitride, tungsten carbide, molybdenumnitride, cobalt, cobalt nitride, ruthenium, palladium, or combinationsthereof. In some embodiments, contact barrier layer 284 includesmultiple layers. For example, contact barrier layer 284 may include afirst sub-layer that includes titanium and a second sub-layer thatincludes titanium nitride. In another example, contact barrier layer 284may include a first sub-layer that includes tantalum and a secondsub-layer that includes tantalum nitride. Contact bulk layer 286includes tungsten, ruthenium, cobalt, copper, aluminum, iridium,palladium, platinum, nickel, low resistivity metal constituent, alloysthereof, or combinations thereof. In the depicted embodiment, contactbulk layer 286 includes tungsten, ruthenium, and/or cobalt. In someembodiments, source/drain contacts 282A, 282B do not include contactbarrier layer 284 (i.e., source/drain contacts 282A, 282B arebarrier-free contacts), such that contact bulk layer 286 physicallycontacts ILD layer 254, CESL 264, ILD layer 252, silicide layer 280,and/or epitaxial source/drain features 240B, 240C. In some embodiments,source/drain contacts 282A, 282B are partially barrier-free, wherecontact barrier layer 284 is disposed between a dielectric layer of MLIfeature 250 and a portion of contact bulk layer 286. In someembodiments, contact bulk layer 286 includes multiple layers.

Turning to FIG. 5 , processing proceeds with forming another dielectriclayer of MLI feature 250. For example, an ILD layer 292 is formed overILD layer 254 and source/drain contacts 282A, 282B. ILD layer 292 issimilar to ILD layer 254. For example, ILD layer 292 is formed by adeposition process, such as CVD, PVD, ALD, HDPCVD, MOCVD, RPCVD, PECVD,LPCVD, ALCVD, APCVD, PEALD, FCVD, other suitable deposition methods, orcombinations thereof. ILD layer 292 includes a dielectric materialincluding, for example, silicon oxide, carbon doped silicon oxide,silicon nitride, silicon oxynitride, TEOS, PSG, BSG, BPSG, FSG, BlackDiamond® (Applied Materials of Santa Clara, Calif.), xerogel, aerogel,amorphous fluorinated carbon, parylene, BCB-based dielectric material,SiLK (Dow Chemical, Midland, Mich.), polyimide, other suitabledielectric material, or combinations thereof. In some embodiments, ILDlayer 292 includes a low-k dielectric material. For example, in thedepicted embodiment, ILD layer 292 includes an ELK dielectric material,such as silicon dioxide (SiO₂) (for example, porous silicon dioxide),silicon carbide (SiC), and/or carbon-doped oxide (for example,SiCOH-based material (having, for example, Si—CH₃ bonds)), each of whichis tuned/configured to exhibit a dielectric constant less than about2.5. In some embodiments, ILD layer 292 may include a multilayerstructure having multiple dielectric materials. In some embodiments, ILDlayer 292 has a thickness t1 of about 10 nm to about 120 nm. Subsequentto the deposition of ILD layer 292, a CMP process and/or otherplanarization process may be performed, such that ILD layer 292 has asubstantially planar surface.

In some embodiments, a CESL 294 is formed over ILD layer 254 andsource/drain contacts 282A, 282B before forming ILD layer 292, such thatCESL 294 is disposed between ILD layer 292 and ILD layer 254 andsource/drain contacts 282A, 282B. CESL 294 is similar to CESL 264. Forexample, CESL 294 includes a material different than ILD layer 292, suchas a dielectric material that is different than the dielectric materialof ILD layer 292 to achieve etching selectivity during subsequentetching processes, such as those used to form interconnect openings thatexpose source/drain contacts 282A, 282B. In other words, CESL 294 andits surrounding layers will include materials having distinct etchingsensitivities to a given etchant. For example, CESL 294 includes amaterial having an etch rate to an etchant that is less than an etchrate of a material of ILD layer 292 to the etchant so that the materialof CESL 294 acts as an etch stop during etching of overlying ILD layer292. The material of CESL 294 may also be configured to promote adhesionbetween CESL 294 and ILD layer 292. In some embodiments, CESL 294includes silicon and nitrogen and/or carbon (for example, SiN, SiCN,SiCON, SiON, SiC, and/or SiCO). In some embodiments, CESL 294 includes ametal oxide layer and/or a metal nitride layer. The metal can includealuminum, hafnium, titanium, copper, manganese, vanadium, other suitablemetal, or combinations thereof. In some embodiments, CESL 294 includesmultiple layers. CESL 294 is formed by CVD, PVD, ALD, HDPCVD, MOCVD,RPCVD, PECVD, LPCVD, ALCVD, APCVD, PEALD, FCVD, other suitabledeposition methods, or combinations thereof. In some embodiments, CESL294 has a thickness t2 of about 1 nm to about 30 nm. Subsequent to thedeposition of CESL 294, a CMP process and/or other planarization processmay be performed, such that CESL 294 has a substantially planar surface.

Turning to FIG. 6 , one or more interconnect openings, such as aninterconnect opening 300A and an interconnect opening 300B, are formedin a dielectric layer (for example, ILD layer 292 and CESL 294) by apatterning process. Interconnect opening 300A extends vertically throughILD layer 292 and CESL 294 to expose source/drain contact 282A.Interconnect opening 300B extends vertically through ILD layer 292 andCESL 294 to expose source/drain contact 282B. Interconnect openings300A, 300B may thus be referred to as via openings. Interconnect opening300A includes a sidewall 302A (defined by ILD layer 292 and CESL 294), asidewall 304A (defined by ILD layer 292 and CESL 294), and a bottom 306A(defined by source/drain contact 282A) that extends between sidewall302A and sidewall 304A. Interconnect opening 300B includes a sidewall302B (defined by ILD layer 292 and CESL 294), a sidewall 304B (definedby ILD layer 292 and CESL 294), and a bottom 306B (defined bysource/drain contact 282B) that extends between sidewall 302B andsidewall 304B. Interconnect openings 300A, 300B have a width x1 definedbetween sidewalls 302A, 302B and sidewalls 304A, 304B, respectively, anda depth d1 defined between a top surface of ILD layer 292 andsource/drain contacts 282A, 282B, respectively. In some embodiments,width x1 is about 10 nm to about 30 nm, and depth d1 is about 10 nm toabout 150 nm (for example, about 20 nm to about 80 nm). As CDs ofinterconnect openings 300A, 300B shrink for advanced IC technologynodes, any subsequently formed metal glue/barrier layer will consumemore space in interconnect openings 300A, 300B, thereby reducing aremainder of interconnect openings 300A, 300B for a subsequently formedmetal fill layer. This can lead to poor metal gap filling, where themetal fill layer is unable to fill a remainder of interconnect openings300A, 300B without forming gaps (or voids), which significantlyincreases contact resistance. In some embodiments, an aspect ratio(e.g., d1/x1) of interconnect openings 300A, 300B is greater than orequal to about 3. In some embodiments, the aspect ratio is about 5 toabout 15. Since aspect ratios greater than or equal to about 3 canintroduce gap filling issues, as described further below, the presentdisclosure reduces the aspect ratio of interconnect openings 300A, 300B(for example, to aspect ratios less than about 3) before forming a metalglue/barrier layer, which can prevent or minimize formation of gapswithin interconnects formed in interconnect openings 300A, 300B. In FIG.6 , each of interconnect openings 300A, 300B has a trapezoidal shape,though the present disclosure contemplates interconnect openings 300A,300B having other shapes, such as a rectangular shape. Sidewalls 302A,304A are tapered, such that a bottom width of interconnect opening 300Aexposing source/drain contact 282A is less than a top width ofinterconnect opening 300A at a top surface of ILD layer 292. Width x1 ofinterconnect opening 300A thus decreases along the z-direction from thetop width of interconnect opening 300A to the bottom width ofinterconnect opening 300A. Sidewalls 302B, 304B are tapered, such that abottom width of interconnect opening 300B exposing source/drain contact282B is less than a top width of interconnect opening 300B at a topsurface of ILD layer 292. Width x1 of interconnect opening 300B thusdecreases along the z-direction from the top width of interconnectopening 300B to the bottom width of interconnect opening 300B. In thedepicted embodiment, interconnect openings 300A, 300B have bottom widthsthat are greater than widths of source/drain contacts 282A, 282B, suchthat interconnect openings 300A, 300B also expose portions of ILD layer292. In some embodiments, bottom widths are smaller or substantially thesame as the widths of source/drain contacts 282A, 282B.

In some embodiments, the patterning process includes performing alithography process to form a patterned mask layer 308 having an opening309A (substantially aligned with source/drain contacts 282A) and anopening 309B (substantially aligned with source/drain contacts 282B)therein over ILD layer 292 and performing an etching process to transfera pattern defined in patterned mask layer 308 to ILD layer 292 and CESL294. The lithography process can include forming a resist layer on ILDlayer 292 (for example, by spin coating), performing a pre-exposurebaking process, performing an exposure process using a mask, performinga post-exposure baking process, and performing a developing process.During the exposure process, the resist layer is exposed to radiationenergy (such as UV light, DUV light, or EUV light), where the maskblocks, transmits, and/or reflects radiation to the resist layerdepending on a mask pattern of the mask and/or mask type (for example,binary mask, phase shift mask, or EUV mask), such that an image isprojected onto the resist layer that corresponds with the mask pattern.Since the resist layer is sensitive to radiation energy, exposedportions of the resist layer chemically change, and exposed (ornon-exposed) portions of the resist layer are dissolved during thedeveloping process depending on characteristics of the resist layer andcharacteristics of a developing solution used in the developing process.After development, the patterned resist layer includes a resist patternthat corresponds with the mask. Alternatively, the exposure process canbe implemented or replaced by other methods, such as masklesslithography, electron-beam writing, and/or ion-beam writing. In someembodiments, the patterned resist layer is patterned mask layer 308. Insuch embodiments, the patterned resist layer is used as an etch mask toremove portions of ILD layer 292 and CESL 294 exposed by openings 309A,309B. In some embodiments, the patterned resist layer is formed over amask layer formed over ILD layer 292 before forming the resist layer,and the patterned resist layer is used as an etch mask to removeportions of the mask layer formed over ILD layer 292, thereby formingpatterned mask layer 308. In such embodiments, the patterned mask layeris used as an etch mask to remove portions of ILD layer 292 and CESL 294exposed by openings 309A, 309B. In some embodiments, after the etchingprocess, patterned mask layer 308 is removed from ILD layer 292 (in someembodiments, by a resist stripping process). In some embodiments,patterned mask layer 308 is removed during etching of ILD layer 292and/or CESL 294.

The etching process can include a dry etching process, a wet etchingprocess, other suitable etching process, or combinations thereof. Forexample, the etching process is a dry etch process, such as an RIEprocess, that uses a fluorine-containing etch gas to remove the materialof ILD layer 292 (e.g., silicon-and-oxygen comprising material) and thematerial of CESL 294 (e.g., silicon-and-nitrogen comprising material) ata higher rate than the materials of patterned mask layer 308 and/orsource/drain contacts 282A, 282B (e.g., metal-comprising material)(i.e., the etchant has a high etch selectivity with respect to ILD layer292 and CESL 294). In some embodiments, the dry etch process isconfigured to generate a fluorine-containing plasma from thefluorine-containing etch gas, such that the dry etch process removes ILDlayer 292 and CESL 294 using plasma-excited fluorine-containing species.The fluorine-containing etch gas includes fluorine (F₂), fluoromethane(e.g., CH₃F), difluoromethane (e.g., CH₂F₂), trifluoromethane (e.g.,CHF₃), tetrafluoromethane (e.g., CF₄), hexafluoroethane (e.g., C₂F₆),sulfur hexafluoride (e.g., SF₆), nitrogen trifluoride (e.g., NF₃), otherfluorine-containing etchant, or combinations thereof. The dry etch can,alternatively or additionally, use a hydrogen-containing etch gas (forexample, H₂ and/or CH₄), a nitrogen-containing etch gas (for example, N₂and/or NH₃), a chlorine-containing etch gas (for example, Cl₂, CHCl₃,CCl₄, and/or BCl₃), an oxygen-containing etch gas (for example, O₂), abromine-containing etch gas (for example, HBr and/or CHBr₃), aniodine-containing etch gas, other suitable etch gas, or combinationsthereof. The dry etch may be configured to generate a plasma from any ofthe etch gases disclosed herein, such that the dry etch usesplasma-excited species for removing ILD layer 292 and CESL 294. In someembodiments, a carrier gas is used to deliver the fluorine-containingetch gas and/or other etch gas. The carrier gas may be an inert gas,such as an argon-containing gas, a helium-containing gas, axenon-containing gas, other suitable inert gas, or combinations thereof.

In some embodiments, the dry etch utilizes an etch gas that includes CF₄to selectively etch ILD layer 292 and CESL 294, In some embodiments, thedry etch utilizes an etch gas that includes CF₄ combined with O₂, N₂,and/or H₂. A flow rate of CF₄, a flow rate of O₂, flow rate of N₂, aflow rate of H₂, a ratio of CF₄ to O₂, N₂, and/or H₂, an etch time, anetch temperature, an etch pressure, and/or an RF power may be tuned toachieve desired etch selectivity. In some embodiments, a flow rate ofCF₄ is about 10 sccm to about 1,000 sccm. In some embodiments, aduration of the dry etch is about 5 seconds to about 50 seconds. In someembodiments, an RF power used for generating a fluorine-containingplasma is about 100 W to about 1,000 W. In some embodiments, a pressuremaintained in a process chamber during the dry etch is about 10 mTorr toabout 1,000 mTorr. In some embodiments, a temperature maintained in theprocess chamber during the dry etch is about 18° C. to about 100° C. Insome embodiments, the etch process is a multi-step (stage) etch processthat includes a first etch step that selectively etches ILD 292 and asecond etch step that selectively etches CESL 294. For example, thefirst etch step is configured to remove ILD layer 292 but not remove orsubstantially remove CESL 294 so that the first etch stops upon reachingCESL 294, while the second etch step is configured to remove CESL 294but not remove or substantially remove ILD 292 and source/drain contacts282A, 282B so that the second etch stops upon reaching source/draincontacts 282A, 282B. Various etch parameters can be tuned to achieveselective etching of ILD layer 292 and CESL 294. For example, for thefirst etch step, an etchant is selected for the etching process thatetches the material of ILD layer 292 at a higher rate than the materialof CESL 294 (i.e., the etchant has a high etch selectivity with respectto the material of ILD layer 292). For the second etch step, an etchantis selected for the etching process that etches the material of CESL 294at a higher rate than the material of ILD layer 292 (i.e., the etchanthas a high etch selectivity with respect to the material of CESL 294).In some embodiments, the first etch step etch and the second etch stepcan use the same etchant but different flow rates and/or concentrationsof constituents of the etchant. In some embodiments, the first etch stepand/or the second etch step may be configured to slightly over etch. Insuch embodiments, the first etch step may partially etch CESL 294 and/orthe second etch step may partially etch source/drain contacts 282A,282B. In some embodiments, the second etch step may include multiplesteps for selectively etching each layer of CESL 294, where each step isconfigured to selectively etch a respective one of the layers of CESL294.

Turning to FIGS. 7A-7C, an etching process is performed to recesssource/drain contacts 282A, 282B. Such process can be referred to as acontact etch back, a contact recess, and/or a plug recess (or etchback). The etching process is a dry etching process, a wet etchingprocess, other suitable etching process, or combinations thereof. In thedepicted embodiment, the contact etch back is a wet etch that utilizes awet etchant solution for removing the material of source/drain contacts282A, 282B (e.g., metal material) at a higher rate than the material ofILD layer 292 (e.g., dielectric material, such as silicon-and-oxygencontaining material) and the material of CESL 294 (e.g., dielectricmaterial, such as silicon-and-nitrogen containing material) (i.e., theetchant has a high etch selectivity with respect to source/draincontacts 282A, 282B). In the depicted embodiment, the wet etchantsolution removes the material of contact bulk layer 286 at a higher ratethan the material of contact barrier layer 284 (i.e., the etchant has ahigh etch selectivity with respect to contact bulk layer 286), such thatthe contact etch back removes contact bulk layer 286 but does not removeor substantially remove contact barrier layer 284. For example, the wetetching process implements a wet etchant solution that includeshydrofluoric acid (HF), nitric acid (HNO₃), hydrochloric acid (HCl),ammonia (NH₄OH), hydrogen peroxide (H₂O₂), water (H₂O), other suitablewet etchant solution constituents, or combinations thereof. In someembodiments, a pH of the wet etchant solution, an etch temperature,and/or an etch time may be tuned to achieve desired etch selectively. Insome embodiments, the wet etchant solution has a pH of about 5.5 toabout 8.5. In some embodiments, a temperature of the wet etchantsolution is about 18° C. to about 100° C. In some embodiments, aduration of the wet etch is about 10 seconds to about 200 seconds. Asdescribed further below, the contact etch back increases a contact areabetween source/drain contacts 282A, 282B and subsequently formed vias,which can improve performance of IC device 200 and/or improve structuralintegrity of the vias and/or interconnect structures including the vias.In some embodiments, the contact etch back partially removes contactbarrier layer 284, such that a thickness of contact barrier layer 284varies along ILD layer 254. In some embodiments, the contact etch backcompletely removes contact barrier layer 284.

After the contact etch back, interconnect opening 300A has an upperportion 310A and a lower portion 311A, and interconnect opening 300B hasan upper portion 310B and a lower portion 311B. Upper portions 310A,310B are configured substantially the same in the X-Z plane and the Y-Zplane. For example, in the X-Z plane, upper portions 310A, 310B aredefined between sidewalls 302A, 302B and sidewalls 304A, 304B,respectively, and have width w1 and depth d1 as described above. In theY-Z plane, upper portions 310A, 310B have a width y1 defined betweensidewalls 312A, 312B and sidewalls 313A, 313B, respectively, and depthd1 defined between a top surface of ILD layer 292 and a bottom surfaceof CESL 294. Sidewalls 312A, 312B and sidewalls 313A, 313B are definedby ILD layer 292 and CESL 294. In some embodiments, width y1 issubstantially the same as width x1 (for example, about 10 nm to about 30nm). In some embodiments, width y1 is greater than or less than widthx1. Upper portions 310A, 310B also have a trapezoidal shape in the Y-Zplane, though the present disclosure contemplates upper portions 310A,310B have other shapes, such as a rectangular shape. Sidewalls 312A,312B and sidewalls 313A, 313B are tapered, such that bottom widths ofupper portions 310A, 310B are less than top widths of upper portions310A, 310B. Width y2 thus decreases along the z-direction from topwidths to bottom widths of upper portions 310A, 310B. The presentdisclosure contemplates embodiments where upper portions 310A, 310B areconfigured differently in the X-Z plane and the Y-Z plane.

Lower portions 311A, 311B are configured differently in the X-Z planeand the Y-Z plane. For example, lower portions 311A, 311B have atrapezoidal shape in the X-Z plane (FIG. 7A) and a semi-circle shape inthe Y-Z plane (FIG. 7B and FIG. 7C), though the present disclosurecontemplates lower portions 311A, 311B having other shapes in the X-Zplane and the Y-Z plane, respectively. In FIG. 7A, lower portion 311A isdefined by a sidewall 314A (defined by contact barrier layer 284), asidewall 315A (defined by contact barrier layer 284), and a bottom 316A(defined by contact bulk layer 286) that extends between sidewall 314Aand sidewall 315A; and lower portion 311B is defined by a sidewall 314B(defined by contact barrier layer 284), a sidewall 315B (defined bycontact barrier layer 284), and a bottom 316B (defined by contact bulklayer 286) that extends between sidewall 314B and sidewall 315B. A widthx2 is defined between sidewalls 314A, 314B and sidewalls 315A, 315B,respectively. In the depicted embodiment, because the contact etch backselectively removes contact bulk layer 286 relative to contact barrierlayer 284, width x2 is less than width x1. In some embodiments, width x2is about 8 nm to about 30 nm. In some embodiments, width x2 is greaterthan or substantially the same as width x1. Sidewalls 314A, 314B andsidewalls 315A, 315B are tapered, such that bottom widths of lowerportions 311A, 311B are less than top widths of lower portions 311A,311B. Width x2 thus decreases along the z-direction from top widths tobottom widths of lower portions 311A, 311B. A depth d2 is definedbetween a top surface of ILD layer 254 and bottoms 316A, 316B,respectively. In some embodiments, depth d2 is less than depth d1. Insome embodiments, depth d2 is less than or equal to about 30 nm.

In FIG. 7B and FIG. 7C, lower portions 311A, 311B further include curvedwalls 318A, 318B (defined by contact bulk layer 286), respectively.Depth d2 is further defined between the top surface of ILD layer 254 andcurved sidewalls 318A, 318B, respectively, in the Y-Z plane.Accordingly, in the depicted embodiment, depth d2 is substantially thesame along the x-direction but varies along the y-direction. A width y2is also defined by curved sidewalls 318A, 318B, respectively. In thedepicted embodiment, because the contact etch back has an isotropicnature (in other words, the contact etch back removes portions ofcontact bulk layer 286 laterally (e.g., along the x-direction and they-direction) and vertically (e.g., along the z-direction)), top widthsof lower portions 311A, 311B are greater than bottom widths of upperportions 310A, 310B. For example, width y2 is greater than width y1. Insome embodiments, width y2 is about 10 nm to about 50 nm. In someembodiments, width y2 is about 10 nm to about 20 nm greater than widthy1. In some embodiments, the contact etch back exposes a portion of abottom surface of CESL 294, such as CESL surfaces 320A and CESL surfaces320B. In some embodiments, a width y3 of exposed CESL surfaces 320A,320B is less than or equal to about 10 nm. Curved sidewalls 318A, 318Balso result in lower portions 311A, 311B having tapered widths in theY-Z plane, such that width y2 decreases along the z-direction from topsto bottoms of lower portions 311A, 311B.

Turning to FIGS. 8A-8C, a via bulk material 330 is formed ininterconnect openings 300A, 300B by a bottom-up deposition process. Viabulk material 330 includes tungsten, tungsten alloy, ruthenium,ruthenium alloy, cobalt, cobalt alloy, copper, copper alloy, aluminum,aluminum alloy, iridium, iridium alloy, palladium, palladium alloy,platinum, platinum alloy, nickel, nickel alloy, other low resistivitymetal constituent and/or alloys thereof, or combinations thereof.Bottom-up deposition process generally refers to a deposition processthat fills an opening from bottom to top (which can also be referred toas a bottom-up fill of an opening). In some embodiments, the bottom-updeposition process is selective CVD, where various parameters of theselective CVD are tuned to selectively grow tungsten, ruthenium, cobalt,or alloys thereof from contact bulk layer 286 and/or contact barrierlayer 284 while limiting (or preventing) growth of tungsten, ruthenium,cobalt, or alloys thereof from ILD layer 292 and/or CESL 294. Thevarious parameters include deposition precursors (for example, metalprecursors and/or reactants), deposition precursor flow rates,deposition temperature, deposition time, deposition pressure, othersuitable deposition parameters, or combinations thereof. In the depictedembodiment, various parameters of the selective CVD are tuned toselectively grow polycrystalline tungsten on contact bulk layer 286using a reduction reaction. In such embodiments, the selective CVDincludes flowing a tungsten-containing precursor (for example, WF_(x)(where x indicates a number of F atoms and x≥1), WCl_(y) (where yindicates a number of Cl atoms and y≥1), other suitabletungsten-containing gas, or combinations thereof) and a reactantprecursor (for example, H₂, other suitable reactant gas, or combinationsthereof) into a process chamber to deposit via bulk material 330 oncontact bulk layer 286 in interconnect openings 300A, 300B. In someembodiments, a carrier gas is used to deliver the tungsten-containingprecursor gas and/or the reactant gas to the process chamber. Thecarrier gas may be an inert gas, such as an argon-containing gas, ahelium-containing gas, a xenon-containing gas, other suitable inert gas,or combinations thereof. In some embodiments, where thetungsten-containing precursor is tungsten hexafluoride (e.g., WF₆),tungsten hexafluoride can be reduced by H₂ as provided for in thefollowing example reduction reaction:WF₆+3H₂→W+6HFIn some embodiments, where the tungsten-containing precursor is tungstenpentachloride (e.g., WCl₅), tungsten pentachloride can be reduced by H₂as provided in the following example reduction reaction:2WCl₅+5H₂→2W+5HClIn some embodiments, the tungsten-containing precursor is mixed with thereactant gas (e.g., H₂) and a ratio of the tungsten-containing precursorto the reactant gas is about 0.1% to about 1.5%. In some embodiments, apressure maintained in the process chamber during the selective CVD isabout 1 Torr to about 50 Torr. In some embodiments, a temperature of ICdevice 200 (for example, substrate 210) during the selective CVD isabout 200° C. to about 400° C. In some embodiments, the variousparameters of the bottom-up deposition process are tuned to minimizevariations in via bulk material 330 in interconnect openings across ICdevice 200, such as those described below and herein. In someembodiments, the bottom-up deposition process is ALD, where variousparameters of the ALD are tuned to selectively grow tungsten, ruthenium,cobalt, or alloys thereof from contact bulk layer 286 and/or contactbarrier layer 284 while limiting (or preventing) growth of tungsten,ruthenium, cobalt, or alloys thereof from ILD layer 292 and/or CESL 294.In some embodiments, multiple ALD cycles are performed to form via bulkmaterial 330. In some embodiments, the bottom-up deposition process ismultiple cycles of deposition/etch steps including, for example,depositing a via bulk material over IC device 200 and then etching backthe via bulk material successively.

As depicted in FIGS. 8A-8C, bottom-up deposition process may notuniformly deposit via bulk material 330 in interconnect openings, suchas interconnect opening 300A and interconnect opening 300B, across ICdevice 200. For example, via bulk material 330 partially fillsinterconnect opening 300A while completely filling interconnect opening300B. In the depicted embodiment, via bulk material 330 completely fillslower portions 311A, 311B, partially fills upper portion 310A, andcompletely fills upper portion 310B. As such, via bulk material 330filling interconnect opening 300A has a height (or thickness) h1 definedbetween source/drain contact 282A (in particular, top surface of contactbulk layer 286) and a top surface 332A of via bulk material 330, and viabulk material 330 filling interconnect opening 300B has a height (orthickness) h2 defined between source/drain contact 282B (in particular,top surface of contact bulk layer 286) and a top surface 332B of viabulk material 330. In some embodiments, height h1 is about 20 nm toabout 50 nm, and height h2 is about 40 nm to about 130 nm. In someembodiments, a ratio of height h1 to height h2 is about 1:2 to about1:7. In some embodiments, a height (growth) variation Δh between viabulk material 330 deposited (grown) in interconnect openings of ICdevice 200, such as interconnect openings 300A, 300B, is about 20 nm toabout 80 nm. In the depicted embodiment, top surface 332A and topsurface 332B have different surface profiles. For example, top surface332A is a substantially planar surface and top surface 332B is asubstantially curved surface, though the present disclosure notes thatboth the planar surface and the curved surface may includeirregularities and/or roughness that naturally arise from the bottom-updeposition process (i.e., the planar surface and the curved surface maynot be completely smooth as depicted). In some embodiments, top surface332A and top surface 332B are substantially planar surfaces orsubstantially curved surfaces. In some embodiments, top surface 332A isa substantially curved surface, while top surface 332B is asubstantially planar surface. A remaining (unfilled) portion ofinterconnect opening 300A has a depth d1′, which is defined between atop surface of ILD layer 292 and top surface 332A of via bulk material330. In some embodiments, depth d1′ is less than about 30 nm. Depth d1′is less than depth d1, which reduces an aspect ratio of interconnectopenings 300A, 300B. In some embodiments, after the bottom-up depositionprocess, an aspect ratio (e.g., d1′/x1) of interconnect openings 300A,300B is less than about 5, and in some embodiments, less than about 3.Reducing the aspect ratio of interconnect openings 300A, 300B canprevent or minimize formation of gaps within interconnects formed ininterconnect openings 300A, 300B. In the depicted embodiment, via bulkmaterial 330 overfills interconnect opening 300B and extends over andabove top surface of ILD layer 292. For example, a portion of via bulkmaterial 330 forms a via rivet head having a height (or thickness) h3defined between the top surface of ILD layer 292 and top surface 332B ofvia bulk material 330. In some embodiments, height h3 is about 5 nm toabout 20 nm. Via rivet head has a width that is greater than width y1.In the depicted embodiment, via bulk material 330 of via rivet headextends over the top surface of ILD layer 292. In such embodiments, awidth y4 of a portion of the top surface of ILD layer 292 covered by viabulk material 330 is about 5 nm to about 15 nm. In some embodiments,width y4 is greater than width y3, such as depicted. In someembodiments, width y4 is less than or substantially equal to width y3.As height h3 of the via rivet head increases, internal stress increaseswithin via bulk material 330, which can lead to cracks during continualgrowth of via bulk material 330 and/or during subsequent processing.Accordingly, in some embodiments, the bottom-up deposition process istuned to constrain height h3 to a predefined height that can minimizeinternal stress of via bulk material 330 while also ensuring that heighth1 is sufficient to reduce an aspect ratio of interconnect openings300A, 300B within a range that enhances metal gap filling. For example,in some embodiments, the bottom-up deposition process is tuned to ensurethat height h3 is less than or equal to 20 nm to minimize internalstress of via bulk material 330 while height h1 is within a targetheight range that achieves depth d1′ less than about 30 nm to provideinterconnect openings 300A, 300B with aspect ratios having improvedmetal gap filling characteristics. In such example, when height h3 isgreater than 20 nm, via bulk material 330 may exhibit degrees ofinternal stress that lead to cracks in via bulk material 330 that cannegatively impact performance of IC device 200.

Turning to FIGS. 9A-9C, a via barrier layer 335 (also referred to as avia liner layer) is formed over via bulk material 330. Via barrier layer335 is formed over the top surface of ILD layer 292, top surfaces 332A,332B of via bulk material 330, and sidewalls 302A, 304A defining aremainder of upper portion 310A. Via barrier layer 335 is formed in andpartially fills interconnect opening 300A. As deposited, via barrierlayer 335 floats above source/drain contact 282A within interconnectopening 300A and does not physically contact source/drain contact 282A.In the depicted embodiment, via barrier layer 335 has a thickness t3over the top surface of ILD layer 292 and top surfaces 332A, 332B of viabulk material 330, and a thickness t4 over sidewalls 302A, 304A defininga remainder of upper portion 310A. In the depicted embodiment, thicknesst3 is greater than thickness t4. For example, thickness t3 is about 4 nmto about 8 nm, and thickness t4 is about 1 nm to about 3 nm. Thicknesst4 greater than about 3 nm may leave insufficient room for asubsequently formed via bulk material, which can lead to gap fillingissues (for example, voids within the vias). In some embodiments, viabarrier layer 335 is conformally deposited over IC device 200, such thatthickness t3 is substantially the same as thickness t4. In someembodiments, thickness t3 is less than thickness t4 depending on processconditions. In some embodiments, via barrier layer 335 is formed overvia bulk material 330 to reduce internal stress and prevent cracks invia bulk material 330. In some embodiments, via bulk material 330 maycompletely fill both interconnect openings 300A, 300B (and, in someembodiments, all interconnect openings across a wafer). In suchembodiments, via barrier layer 335 (and subsequently formed via bulkmaterial described below) may still be formed to reduce internal stressand prevent cracks in via bulk material 330, for example, during aplanarization process. Via barrier layer 335 includes a material thatpromotes adhesion between a dielectric material (here, ILD layer 292)and a subsequently formed metal material for filling a remainder ofinterconnect opening 300A. For example, via barrier layer 335 includestitanium, titanium alloy, tantalum, tantalum alloy, cobalt, cobaltalloy, ruthenium, ruthenium alloy, molybdenum, molybdenum alloy,tungsten, tungsten alloy, other suitable constituent configured topromote and/or enhance adhesion between a metal material and adielectric material, or combinations thereof. In the depictedembodiment, via barrier layer 335 includes tantalum and nitrogen (forexample, tantalum nitride), titanium and nitrogen (for example, titaniumnitride), tungsten and nitrogen (for example, tungsten nitride), ortungsten. In some embodiments, via barrier layer 335 includes multiplelayers. For example, via barrier layer 335 includes a first sub-layerthat includes titanium and a second sub-layer that includes titaniumnitride disposed over the first sub-layer. In another example, viabarrier layer 335 includes a first sub-layer that includes tantalum anda second sub-layer that includes tantalum nitride.

Via barrier layer 335 is deposited by PVD, CVD, ALD, electroplating,electroless plating, other suitable deposition process, or combinationsthereof. In some embodiments, via barrier layer 335 is a titaniumnitride layer formed by performing PVD to deposit a titanium layer overIC device 200 and then performing a nitrogen plasma treatment on thetitanium layer. In some embodiments, a pressure maintained in a processchamber during the PVD is about 10 mTorr to about 1 Torr. In someembodiments, a temperature maintained in the process chamber during thePVD is about 300° C. to about 450° C. In some embodiments, the PVD is aradio frequency (RF) sputtering process. In such embodiments, the RFsputtering process may implement an RF power of about 1 kilowatt (kW) toabout 2 kW. In some embodiments, the nitrogen plasma treatment includesexposing the titanium layer to a nitrogen-containing plasma (i.e., thetitanium layer is bombarded with plasma-excited nitrogen-containingspecies to drive nitrogen into the titanium layer). In some embodiments,via barrier layer 335 includes a titanium nitride layer disposed over atitanium layer, where the titanium layer is formed by performing PVD andthe titanium nitride layer is formed by performing CVD. In someembodiments, a pressure maintained in a process chamber during the PVDis about 10 mTorr to about 150 mTorr. In some embodiments, a temperaturemaintained in the process chamber during the PVD is about 400° C. toabout 500° C. In embodiments where the PVD is an RF sputtering process,the PVD can implement an RF power of about 3 kW to about 5 kW. In someembodiments, the CVD implements a titanium-containing precursor, such astetrakis(dimethylamino)titanium (TDMAT). In some embodiments, a pressuremaintained in a process chamber during the CVD is about 1 Torr to about5 Torr. In some embodiments, a temperature maintained in the processchamber during the CVD is about 100° C. to about 500° C. In someembodiments, via barrier layer 335 includes a tungsten-comprising layerformed by CVD, where the CVD implements a tungsten-containing precursor,such as tungsten hexacarbonyl (W(CO)₆). In some embodiments, the CVDprocess implements a mixture of the tungsten-containing precursor withhydrogen (H₂), silane (SiH₄), and/or borane (for example, diborane(B₂H₆)).

Turning to FIGS. 10A-10C, a via bulk material 340 is formed over viabarrier layer 335. Via bulk material 340 is formed in and fills aremainder of upper portion 310A of interconnect opening 300A. In thedepicted embodiment, via bulk material 340 is disposed over and coversthe top surface of ILD layer 292 and top surface 332B of via bulkmaterial 330. In some embodiments, via bulk material 340 does not coveror only partially covers top surface 332B depending on a thickness ofvia bulk material 340. Via bulk material 340 includes tungsten, tungstenalloy, ruthenium, ruthenium alloy, cobalt, cobalt alloy, copper, copperalloy, aluminum, aluminum alloy, iridium, iridium alloy, palladium,palladium alloy, platinum, platinum alloy, nickel, nickel alloy, otherlow resistivity metal constituent and/or alloys thereof, or combinationsthereof. In the depicted embodiment, via bulk material 340 and via bulkmaterial 330 include the same metal. For example, via bulk material 340and via bulk material 330 include tungsten. In some embodiments, viabulk material 340 and via bulk material 330 include different metals. Inthe depicted embodiment, via bulk material 340 and via bulk material 330are formed by different processes. For example, a blanket depositionprocess, such as blanket CVD, is performed to deposit via bulk material340 over via barrier layer 335. The blanket deposition process depositsvia bulk material 340 over an entirety of exposed surfaces of IC device200. In some embodiments, the blanket CVD includes flowing atungsten-containing precursor (for example, WF₆ or WCl₅ and a reactantprecursor (for example, H₂, other suitable reactant gas, or combinationsthereof)) into a process chamber. In some embodiments, a carrier gas isused to deliver the tungsten-containing precursor gas and/or thereactant gas to the process chamber. The carrier gas may be an inertgas, such as an argon-containing gas, a helium-containing gas, axenon-containing gas, other suitable inert gas, or combinations thereof.In some embodiments, a pressure maintained in a process chamber duringthe blanket CVD is about 1 Torr to about 500 Torr. In some embodiments,a temperature maintained in the process chamber during the blanket CVDis about 200° C. to about 400° C. In some embodiments, the blanketdeposition process is PVD, ALD, electroplating, electroless plating,other suitable deposition process, or combinations thereof.

Turning to FIGS. 11A-11C, a CMP process and/or other planarizationprocess is performed to remove excess via bulk material 340, via barrierlayer 335, and via bulk material 330 (such as that disposed over the topsurface of ILD layer 292), resulting in a via 350A and a via 350B of MLIfeature 250. Via 350A electrically couples and/or physically couplessource/drain contact 282A to a conductive feature of MLI feature 250,such as a first conductive line of a metal layer of MLI feature 250 (forexample, a metal one (M1) layer), and via 350B electrically couplesand/or physically couples source/drain contact 282B to a conductivefeature of MLI feature 250, such as a second conductive line of a metallayer of MLI feature 250 (for example, the M1 layer). Vias 350A, 350Bcan thus be referred to as vias-to-contacts. The CMP process canplanarize top surfaces of vias 350A, 350B, such that the top surface ofILD layer 292 and the top surfaces of vias 350A, 350B are substantiallyplanar surfaces. Vias 350A, 350B include via bulk material 340, viabarrier layer 335, and/or via bulk material 330 that remains ininterconnect openings 300A, 300B after the planarization process, whichare referred to respectively as a via bulk layer 340′, a via barrierlayer 335′, and a via bulk layer 330′. Via 350A has a lower via portionAL and an upper via portion AU, and via 350B has a lower via portion BLand an upper via portion BU. Lower via portions AL, BL extend intosource/drain contacts 282A, 282B, respectively. In particular, lower viaportions AL, BL extend from the top surface of ILD layer 254 to the topsurface of contact barrier layer 286 of source/drain contacts 282A,282B, respectively. Lower via portions AL, BL have width x2 along thex-direction, width y2 along the y-direction, and a thickness t5 alongthe z-direction. In the depicted embodiment, thickness t5 is about equalto depth d2 (for example, thickness t5 is less than or equal to about 10nm). Upper via portions AU, BU extend through ILD layer 292 and CESL294, for example, from the top surface of ILD layer 292 to the topsurface of ILD layer 254. Upper via portions AU, BU have width x1 alongthe x-direction, width y1 along the y-direction, and a thickness t6along the z-direction. In the depicted embodiment, thickness t6 is aboutequal to depth d1 (for example, thickness t6 is about 20 nm to about 80nm). In the depicted embodiment, in the X-Z plane, a width of lower viaportions AL, BL is less than a width of upper via portions AU, BU (i.e.,width x2 is less than width x1), and in the Y-Z plane, a width of lowervia portions AL, BL is greater than a width of upper via portions AU, BU(i.e., width y2 is greater than width y1). In some embodiments, thewidth of lower via portions AL, BL is greater than or about equal to thewidth of upper via portions AU, BU in the X-Z plane. In someembodiments, the width of lower via portions AL, BL is less than orabout equal to the width of upper via portions AU, BU in the Y-Z plane.

Lower via portion AL and lower via portion BL are substantially thesame. For example, each of lower via portions AL, BL include a lowerportion of via bulk layer 330′ that physically contacts contact barrierlayer 284 and contact bulk layer 286 of source/drain contacts 282A,282B, respectively. In the X-Z plane, the lower portions of via bulklayer 330′ have tapered, substantially planar side surfaces thatinterface with contact barrier layer 284 of source/drain contacts 282A,282B, respectively, and substantially planar bottom surfaces thatinterface with contact bulk layer 286 of source/drain contacts 282A,282B, respectively. Contact barrier layer 284 separates the tapered,substantially planar side surfaces of the lower portions of via bulklayer 330′ from ILD layer 254. In the Y-Z plane, the lower portions ofvia bulk layer 330′ have substantially curved surfaces that interfacewith contact bulk layer 286 of source/drain contacts 282A, 282B,respectively. Contact bulk layer 286 and contact barrier layer 284separate the substantially curved surfaces of the lower portions of viabulk layer 330′ from ILD layer 254. Lower via portions AL, BL increase acontact area between vias 350A, 350B and source/drain contacts 282A,282B, which reduces resistance between vias 350A, 350B and source/draincontacts 282A, 282B, thereby improving overall performance of IC device200. For example, instead of vias 350A, 350B having a single interface,for example, between bottom surfaces of vias 350A, 350B and top surfacesof source/drain contacts 282A, 282B, respectively, lower via portionsAL, BL are surrounded by source/drain contacts 282A, 282B, respectively,and have multiple interfaces (for example, bottom interfaces andsidewall interfaces) with source/drain contacts 282A, 282B,respectively. Increased contact area provided by lower via portions AL,BL also improves mechanical strength, and thus structural stability, ofvias 350A, 350B. Further, in the Y-Z plane, portions of via bulk layer330′ that extend under and engage CESL surfaces 320A, 320B,respectively, secure vias 350A, 350B to source/drain contacts 282A,282B, further improving mechanical strength, and thus structuralstability, of vias 350A, 350B.

Because growth (and thus heights) of via bulk material 330 varies ininterconnect openings of IC device 200, via bulk layer 335′ ispositioned randomly within vias of IC device 200, resulting in vias tosource/drain contacts having different configurations across IC device200. For example, in FIGS. 11A-11C, because via bulk material 330partially filled interconnect opening 300A and completely filledinterconnect opening 300B, upper via portion AU of via 350A includes viabulk layer 330′, via barrier layer 335′, and via bulk layer 340′, whileupper via portion BU of via 350B includes only via bulk layer 330′.Upper via portion AU can be separated into an upper via portion AU-1 andan upper via portion AU-2. Upper via portion AU-1 is similar to uppervia portion BU, except a thickness of upper via portion AU-1 is lessthan a thickness of upper via portion BU. For example, each of upper viaportions AU-1, BU include an upper portion of via bulk layer 330′. Theupper portion of via bulk layer 330′ has thickness t6 in via 350B,whereas the upper portion of via bulk layer 330′ has a thickness t7 thatis less than thickness t6 in via 350A. In the X-Z plane and the Y-Zplane, the upper portions of via bulk layer 330′ have tapered,substantially planar side surfaces that physically contact and interfacewith ILD layer 292 and CESL 294. No barrier (or liner) layer thus existsbetween upper via portions AU-1, BU and a dielectric layer of MLIfeature 250 (for example, CESL 294 and ILD layer 292). In the depictedembodiment, via 350B can be referred to as a barrier-free via because nobarrier layer is disposed between via bulk layer 330′ and ILD layer 292or CESL 294. In contrast, since upper via portion AU-1 does not have abarrier layer disposed between via bulk layer 330′ and ILD layer 292 orCESL 294, but upper via portion AU-2 includes via barrier layer 335′disposed between via bulk layer 340 and ILD 292, via 350A can bereferred to as a partial barrier-free via. In FIGS. 11A-11C, via barrierlayer 335′ is further disposed between via bulk layer 340′ and via bulklayer 330′, such that via barrier layer 335′ floats within via 350A anddoes not physically contact source/drain contact 282A. Upper via portionAU-2 has a thickness t8 along the z-direction. In the depictedembodiment, thickness t8 is about equal to depth d1′ (for example,thickness t8 is about 1 nm to about 50 nm). In some embodiments, a ratioof thickness t8 to width x1 (e.g., t8/x1) of upper via portion AU-2 isless than about 5, and in some embodiments, is less than about 3.Thickness t8 may be greater or less than thickness t7 depending ongrowth of via bulk material 330.

In another example, such as that depicted in FIGS. 17A-17C, becausegrowth (and thus heights) of via bulk material 330 varies ininterconnect openings of IC device 200, instead of via bulk material 330partially filling interconnect opening 300A and completely fillinginterconnect opening 300B, via bulk material 330 partially fills bothinterconnect opening 300A and interconnect opening 300B but hasdifferent thicknesses. In such embodiments, upper via portion AU of via350A and upper via portion BU of via 350B each include via bulk layer330′, via barrier layer 335′, and via bulk layer 340′. Upper via portionAU can be separated into an upper via portion AU-1 and an upper viaportion AU-2, and upper via portion BU can be separated into an uppervia portion BU-1 and an upper via portion BU-2. Upper via portion AU-1is similar to upper via portion BU-1, except a thickness of upper viaportion AU-1 is less than a thickness of upper via portion BU-1. Forexample, each of upper via portions AU-1, BU-1 include an upper portionof via bulk layer 330′, but the upper portion of via bulk layer 330′ hasthickness t9 in via 350B and the upper portion of via bulk layer 330′has thickness t7, which is less than thickness t9. Both thickness t7 andthickness t9 are less than thickness t6 of upper via portions AU, BU. Inthe X-Z plane and the Y-Z plane, the upper portions of via bulk layer330′ have tapered, substantially planar side surfaces that physicallycontact and interface with ILD layer 292 and CESL 294. No barrier (orliner) layer thus exists between upper via portions AU-1, BU-1 and adielectric layer of MLI feature 250 (for example, CESL 294 and ILD layer292). In the depicted embodiment, since upper via portions AU-1, BU-1 donot have a barrier layer disposed between via bulk layer 330′ and ILDlayer 292 or CESL 294, but upper via portions AU-2, BU-2 include viabarrier layer 335′ disposed between via bulk layer 340 and ILD 292, vias350A, 350B can be referred to as a partial barrier-free via. In FIGS.17A-17C, via barrier layer 335′ is further disposed between via bulklayer 340′ and via bulk layer 330′, such that via barrier layer 335′floats within via 350A and via 350B and does not physically contactsource/drain contacts 282A, 282B. Upper via portion BU-2 has a thicknesst10 that is less than thickness t8 of upper via portion AU-2, such thatvia barrier layer 335′ is positioned within via 350A differently thanvia barrier layer 335′ is positioned within via 350B. For example, inthe depicted embodiment, a distance between a top surface of substrate210 and via barrier layer 335′ of via 350A (in particular, a bottomsurface of via barrier layer 335′) is less than a distance between thetop surface of substrate 210 and via barrier layer 335′ of via 350B (inparticular, a bottom surface of via barrier layer 335′). In someembodiments, in FIGS. 17A-17C, a ratio of thickness t8 to width x1(e.g., t8/x1) of upper via portion AU-2 and a ratio of thickness t10 towidth x1 (e.g., t10/x1) of upper via portion BU-2 are each less thanabout 5, and in some embodiments, is less than about 3. In the depictedembodiment, the ratio of thickness t8 to width x1 is greater than theratio of thickness t10 to width x1.

Turning to FIGS. 12-15 , a via is formed to one or more gate structuresof IC device 200, such as gate structure 230B. Turning to FIG. 12 , aninterconnect opening 360 is formed in a dielectric layer by a patterningprocess, such as those described herein. Interconnect opening 360extends vertically through ILD layer 292, CESL 294, ILD layer 254, andCESL 264 to metal gate stack 232 of gate structure 230B. Interconnectopening 360 may be referred to as a gate contact (plug) opening.Interconnect opening 360 includes a sidewall 362 (defined by ILD layer292, CESL 294, ILD layer 254, and CESL 264), a sidewall 364 (defined byILD layer 292, CESL 294, ILD layer 254, and CESL 264), and a bottom 366(defined by metal gate stack 232) that extends between sidewall 362 andsidewall 364. In FIG. 12 , interconnect opening 360 has a trapezoidalshape, though the present disclosure contemplates interconnect opening360 having other shapes, such as a rectangular shape. Sidewalls 362, 364are tapered, such that a bottom width of interconnect opening 360,exposing metal gate stack 232 of gate structure 230B, is less than a topwidth of interconnect opening 360 at a top surface of ILD layer 292. Awidth of interconnect opening 360 thus decreases along the z-directionfrom top to bottom of interconnect opening 360. In some embodiments, thewidth of interconnect opening 360 increases along the z-direction. Insome embodiments, the width of interconnect opening 360 is substantiallythe same along the z-direction.

In some embodiments, the patterning process includes performing alithography process to form a patterned mask layer 365 having an opening367 (substantially aligned with metal gate stack 232 of gate structure232B) and performing an etching process to transfer a pattern defined inpatterned mask layer 365 to an underlying dielectric layer (here, ILDlayer 292, CESL 294, ILD layer 254, and CESL 264). The patterningprocess for forming patterned mask layer 365 and interconnect opening360 may be similar to the patterning process for forming patterned masklayer 278 and interconnect openings 300A, 300B as described above. Insome embodiments, patterned mask layer 365 is a patterned resist layer.In such embodiments, the patterned resist layer is used as an etch maskto remove portions of the underlying dielectric layer exposed by opening367. In some embodiments, the patterned resist layer is formed over amask layer formed over the underlying dielectric layer before formingthe resist layer, and the patterned resist layer is used as an etch maskto remove portions of the mask layer formed over the underlyingdielectric layer, thereby forming patterned mask layer 365. In suchembodiments, the patterned mask layer is used as an etch mask to removeportions of the underlying dielectric layer exposed by opening 367.Various selective etching processes can be performed to forminterconnect opening 360. For example, the etching process can include afirst etch that selectively etches ILD layer 292 relative to patternedmask layer 365 and CESL 294, such that the first etch stops uponreaching CESL 294; a second etch that selectively etches CESL 294relative to ILD layers 294, 254, such that the second etch stops uponreaching ILD layer 254; a third etch that selectively etches ILD layer254 relative to CESLs 294, 264, such that the third etch stops uponreaching CESL 264; and a fourth etch that selectively etches CESL 264relative to ILD layers 292, 254 and metal gate stack 232 of gatestructure 230B, such that the fourth etch stops upon reaching metal gatestack 232. In some embodiments, the first etch, the second etch, thethird etch, and the fourth etch may be configured to slightly over etchas described herein. In some embodiments, the etching process mayinclude multiple steps for etching CESLs 294, 264. In some embodiments,the etching process implements an etchant with low etch selectivitybetween ILD layers 292, 254 and CESLs 294, 264, such that interconnectopening 360 is formed in a single etch step based on, for example, time.In some embodiments, after the etching process, patterned mask layer 365is removed from ILD layer 292 (in some embodiments, by a resiststripping process). In some embodiments, patterned mask layer 365 isremoved during etching of ILD layer 292, CESL 294, ILD layer 254, and/orCESL 264.

Turning to FIG. 13 , a via barrier layer 370 (also referred to as a vialiner layer) is formed over IC device 200. For example, via barrierlayer 370 is formed over the top surface of ILD layer 292, top surfacesof vias 350A, 350B, sidewalls 362, 364 of interconnect opening 360A, andbottom 366 of interconnect opening 360. Via barrier layer 370 is formedin and partially fills interconnect opening 360. In the depictedembodiment, via barrier layer 370 has a thickness t9 over the topsurface of ILD layer 292 and top surfaces of vias 350A, 350B, and athickness t10 over sidewalls 362, 364 of interconnect opening 360. Inthe depicted embodiment, thickness t9 is greater than thickness t10. Forexample, thickness t9 is about 4 nm to about 8 nm, and thickness t10 isabout 1 nm to about 3 nm. In some embodiments, via barrier layer 370 isconformally deposited over IC device 200, such that thickness t9 issubstantially the same as thickness t10. In some embodiments, thicknesst9 is less than thickness t10 depending on process conditions. Viabarrier layer 370 includes a material that promotes adhesion between adielectric material (here, ILD layer 292, CESL 294, ILD layer 254, andCESL 264) and a subsequently formed metal material for filling aremainder of interconnect opening 360. For example, via barrier layer370 includes titanium, titanium alloy, tantalum, tantalum alloy, cobalt,cobalt alloy, ruthenium, ruthenium alloy, molybdenum, molybdenum alloy,tungsten, tungsten alloy, other suitable constituent configured topromote and/or enhance adhesion between a metal material and adielectric material, or combinations thereof. In the depictedembodiment, via barrier layer 370 includes tantalum and nitrogen (forexample, tantalum nitride), titanium and nitrogen (for example, titaniumnitride), tungsten and nitrogen (for example, tungsten nitride), ortungsten. In some embodiments, via barrier layer 370 includes multiplelayers. For example, via barrier layer 370 includes a first sub-layerthat includes titanium and a second sub-layer that includes titaniumnitride disposed over the first sub-layer. In another example, viabarrier layer 370 includes a first sub-layer that includes tantalum anda second sub-layer that includes tantalum nitride. Via barrier layer 370is deposited by PVD, CVD, ALD, electroplating, electroless plating,other suitable deposition process, or combinations thereof. In someembodiments, via barrier layer 370 is similar to via barrier layer 335and is formed using processes similar to those described above forforming via barrier layer 335.

Turning to FIG. 14 , a via bulk layer 375 is formed over via barrierlayer 370. Via bulk layer 375 is formed in and fills a remainder ofinterconnect opening 360. In the depicted embodiment, via bulk layer 375is disposed over and covers the top surface of ILD layer 292 and topsurfaces of vias 350A, 350B. Via bulk layer 375 includes tungsten,tungsten alloy, ruthenium, ruthenium alloy, cobalt, cobalt alloy,copper, copper alloy, aluminum, aluminum alloy, iridium, iridium alloy,palladium, palladium alloy, platinum, platinum alloy, nickel, nickelalloy, other low resistivity metal constituent and/or alloys thereof, orcombinations thereof. In the depicted embodiment, via bulk layer 375includes tungsten. Via bulk layer 375 is deposited by PVD, CVD, ALD,electroplating, electroless plating, other suitable deposition process,or combinations thereof. In some embodiments, via bulk layer 375 issimilar to via bulk material 340 and is formed using processes similarto those described above for forming via bulk material 340. For example,a blanket deposition process, such as blanket CVD, is performed todeposit via bulk material 375 over via barrier layer 370.

Turning to FIG. 15 , a CMP process and/or other planarization process isperformed to remove excess via bulk layer 375 and via barrier layer 370(such as that disposed over the top surface of ILD layer 292 and the topsurfaces of vias 350A, 350B), resulting in a via 380 of MLI feature 250.Via 380 includes via bulk material 375 and via barrier layer 370 thatremains in interconnect opening 360 after the planarization process,which are referred to respectively as a via bulk layer 375′ and a viabarrier layer 370′. Via 380 electrically couples and/or physicallycouples metal gate stack 232 of gate structure 230B to a conductivefeature of MLI feature 250, such as a third conductive line of a metallayer of MLI feature 250 (for example, the M1 layer). Via 380 can thusbe referred to as a via-to-gate. Via 380 extends through ILD layer 292,CESL 294, ILD layer 254, and CESL 264. Via 380 has a width x4 along thex-direction and a thickness t11 along the z-direction. In the depictedembodiment, thickness t11 is greater than thickness t6 of vias 350A,350B, and width x4 is less than width x1 of vias 350A, 350B. The CMPprocess can planarize top surfaces of via 380, such that the top surfaceof ILD layer 292, the top surfaces of vias 350A, 350B, and the topsurface of via 380 are substantially planar surfaces.

Turning to FIG. 16 , fabrication of MLI feature 250 can continue, forexample, by forming additional dielectric layers and metal layers of MLIfeature 250. For example, a CESL 390 (similar to CESLs 262, 264, 294) isformed over ILD layer 292, vias 350A, 350B, and via 380, an ILD layer392 (similar to ILD layers 252, 254, 292) is formed over CESL 390, andone or more conductive lines (for example, a conductive line 394A, aconductive line 394B, and a conductive line 394C) are formed in ILDlayer 392 and CESL 390. Conductive lines 394A-394C include tungsten,ruthenium, cobalt, copper, aluminum, iridium, palladium, platinum,nickel, other low resistivity metal constituent, alloys thereof, orcombinations thereof. In some embodiments, forming conductive lines394A-394C includes performing a lithography and etching process to formthree interconnect openings in ILD layer 392 and CESL 390 (here,exposing via 350A, via 350B, and via 380), filling the interconnectopenings with a conductive material, and performing a planarizationprocess that removes excess conductive material, such that theconductive material and ILD layer 392 have substantially planarsurfaces. The interconnect openings have sidewalls defined by ILD layer392 (along with CESL 390) and a bottom defined by a respective via. Theconductive material is formed by a deposition process (for example, PVD,CVD, ALD, and/or other suitable deposition process) and/or annealingprocess. In some embodiments, conductive lines 394A-394C include a bulklayer (also referred to as a conductive plug). In some embodiments,conductive lines 394A-394C includes a barrier layer, an adhesion layer,and/or other suitable layer disposed between the bulk layer and ILDlayer 392 (along with CESL 390). In such embodiments, the barrier layerand/or the adhesion layer conform to the interconnect openings, suchthat the barrier layer and/or the adhesion layer are disposed on ILDlayer 392 (along with CESL 390) and the bulk layer is disposed on thebarrier layer and/or the adhesion layer. In some embodiments, thebarrier layer, the adhesion layer, and/or other suitable layer includetitanium, titanium alloy (for example, TiN), tantalum, tantalum alloy(for example, TaN), other suitable constituent, or combinations thereof.In the depicted embodiment, conductive lines 394A-394C have arectangular-shaped cross-section. For example, conductive line 394A-394Chas a substantially planar bottom surface, a substantially planar topsurface, and substantially planar sidewalls. In some embodiments,sidewalls of conductive lines 394A-394C are tapered, such that athickness of conductive lines 394A-394C decreases from a top surface ofILD layer 390 to the top surface of ILD layer 292. In furtherance of thedepicted embodiment, conductive line 394A physically contacts via 350A,conductive line 394B physically contacts via 380, and conductive line394C physically contacts via 350B. In some embodiments, conductive line394A and conductive line 394B respectively electrically couple epitaxialsource/drain features 240B, 240C to a first voltage, and conductive line394C electrically couples metal gate 232 to a second voltage.

In FIG. 16 , MLI feature 250 includes a metal-0 (M0) layer (whichincludes source/drain contacts 282A, 282B), a metal-1 (M1) layer (whichincludes conductive lines 394A-394C), and a via-0 (V0) layer (whichincludes vias 350A, 350B and via 380) that physically and electricallyconnects the M0 layer to the M1 layer. The M0 layer is a lowest,bottommost contact/metal layer of MLI feature 250 and physicallycontacts device features at a substrate level (for example, source/drainfeatures and/or gate structures), the V0 layer is a lowest, bottommostvia layer of MLI feature 250, and the M1 layer is a second lowest,bottommost contact/metal layer of MLI feature 250. The M0 layer can alsobe referred to as a contact layer, local interconnect layer, ordevice-level contact layer. In the depicted embodiment, via 380 directlyand physically contacts gate structure 230B. In some embodiments, the M0layer further includes a gate contact disposed between via 380 and gatestructure 230B, where via 380 physically contacts the gate contact andthe gate contact physically contacts gate structure 230B. In suchembodiments, the gate contact may be configured similar to source/draincontacts 282A, 282B. For example, the gate contact may include a contactbulk layer, such as contact bulk layer 286, disposed over a contactbarrier layer, such as contact barrier layer 284. In such embodiments,in contrast to contact bulk layer 286 of source/drain contacts 282A,282B, the contact bulk layer of the gate contact is not recessed whenforming via 280, such that the contact barrier layer of the gate contactis not disposed along a portion of sidewalls of via 280. In someembodiments, the present disclosure contemplates recessing the contactbulk layer of the gate contact before forming via 280, such that via 280extends into the gate contact similar to vias 350A, 350B. Though notdepicted, it is understood that MLI feature 250 can include additionalmetal layers and/or via layers formed over the M1 layer, such as a via-2(V2) layer disposed over the M1 layer, a metal-2 (M2) layer disposedover the V2 layer, and so forth to provide desired electrical connectionfor IC device 200. Fabrication can then continue fabrication of MLIfeature 250. For example, additional levels of MLI feature 250 can beformed over the M1 layer, such as an M2 layer to an Mn layer, where nrepresents a number of metal layers in MLI feature 250 and each of M2layer to Mn layer include conductive lines, similar to conductive lines394A-394C disposed in a dielectric material. Vias, similar to vias 350A,350B, can be fabricated to connect directly adjacent metal layers, suchas M2 layer to M3 layer. In some embodiments, vias may connectnon-adjacent metal layers.

From the foregoing description, it can be seen that vias described inthe present disclosure offer advantages over conventional vias. It isunderstood, however, that other embodiments may offer additionaladvantages, and not all advantages are necessarily disclosed herein, andthat no particular advantage is required for all embodiments. Oneadvantage is that vias-to-contacts and vias-to-gates are provided withdifferent configurations (in particular, different interfaceconfigurations) to optimize contact resistance reduction. For example,vias-to-contacts are at least partially barrier free to increasedimensions of vias-to-contacts openings, which improves metal filling ofthe via-to-contact openings (in some embodiments, reducing oreliminating voids within the vias-to-contacts) and reducesvia-to-contact resistance, while vias-to-gates have a via barrier layerto ensure minimal via-to-gate resistance. The present disclosure notesthat, when via barrier layers are implemented in both vias-to-contactsand vias-to-gates, interfaces in vias-to-contacts (e.g., between viabarrier layers and source/drain contacts) cannot achieve the sameresistance reduction as interfaces in vias-to-gates (e.g., between viabarrier layers and metal gates), and that any resistance reductionprovided by the via barrier layers in vias-to-contacts does notsignificantly outweigh risks of voids forming in vias-to-contacts whenthe via barrier layers are incorporated therein. Removing the viabarrier layers from the vias-to-contacts thus lowers any risk of voidformation and thus minimizes vias-to-contacts resistance as well as, ifnot more than, vias-to-contacts having via barrier layers. For example,vias-to-contacts having via barrier layers may have voids therein,leading to increasing resistance of vias-to-contacts more than providedby any lowering of the resistance provided by incorporating the viabarrier layers. In another example, via bulk material ofvias-to-contacts formed by a bottom-up deposition process, as describedherein, exhibit low resistance characteristics. In yet another example,forming a via barrier layer and an additional via bulk layer after thebottom-up deposition process ensures complete filling of via-to-contactopenings (accounting for growth variations that arise from the bottom-updeposition process) and/or minimizes internal stress of the via bulklayer and thus reduces or eliminates cracking of the via bulk layerduring planarization and/or other subsequent processing. The disclosedvia fabrication methods thus account for various via interfacerequirements and/or considerations.

The present disclosure provides for many different embodiments. Anexemplary interconnect structure includes a first source/drain contactand a second source/drain contact disposed in a dielectric layer. Thefirst source/drain contact physically contacts a first source/drainfeature and the second source/drain contact physically contacts a secondsource/drain feature. A first via having a first via layerconfiguration, a second via having a second via layer configuration, anda third via having a third via layer configuration are disposed in thedielectric layer. The first via and the second via extend into andphysically contact the first source/drain contact and the secondsource/drain contact, respectively. A first thickness of the first viaand a second thickness of the second via are the same. The third viaphysically contacts a gate structure, which is disposed between thefirst source/drain contact and the second source/drain contact. In someembodiments, the first via includes a first upper portion disposed overa first lower portion and the second via includes a second upper portiondisposed over a second lower portion. In such embodiments, thedielectric layer physically contacts sidewalls of the first upperportion and the first source/drain contact physically contacts sidewallsof the first lower portion and the dielectric layer physically contactssidewalls of the second upper portion and the second source/draincontact physically contacts sidewalls of the second lower portion. Insome embodiments, each of the first via and the second via have a firstcross-sectional profile along a first direction and a secondcross-sectional profile along a second direction. The first direction isdifferent than the second direction and the first cross-sectionalprofile is different than the second cross-sectional profile. In someembodiments, each of the first via and the second via have a firstbottom interface profile with the first source/drain contact and thesecond source/drain contact, respectively, along a first direction and asecond bottom interface profile with the first source/drain contact andthe second source/drain contact, respectively, along a second direction.The first direction is different than the second direction, the firstbottom interface profile is defined by a substantially planar surface,and the second bottom interface profile is defined by a substantiallycurved surface.

In some embodiments, the dielectric layer includes a first interleveldielectric layer, a second interlevel dielectric layer disposed over thefirst interlevel dielectric layer, and a contact etch stop layerdisposed between the first interlevel dielectric layer and the secondinterlevel dielectric layer. The first source/drain contact and thesecond source/drain contact are disposed in and extend through the firstinterlevel dielectric layer. The first via and the second via aredisposed in and extend through the second interlevel dielectric layerand the contact etch stop layer. In such embodiments, a first portion ofthe first via extending into the first source/drain contact physicallycontacts a bottom surface of the contact etch stop layer and a secondportion of the second via extending into the second source/drain contactphysically contacts the bottom surface of the contact etch stop layer.

In some embodiments, the first via having the first via layerconfiguration includes a first via bulk layer, a second via bulk layerdisposed over the first via bulk layer, and a via barrier layer disposedbetween the first via bulk layer and the second via bulk layer. Thefirst via bulk layer physically contacts the dielectric layer and thevia barrier layer is further disposed between the second via bulk layerand the dielectric layer. In furtherance of such embodiments, the secondvia having the second via layer configuration includes a third via bulklayer that physically contacts the dielectric layer. The first via bulklayer and the third via bulk layer include a same material, the thirdvia bulk layer has the second thickness, and the first via bulk layerhas a third thickness that is less than the first thickness. In someembodiments, the first source/drain contact includes a first contactbarrier layer and a first contact bulk layer, and the secondsource/drain contact includes a second contact barrier layer and asecond contact bulk layer. In some embodiments, a first portion of thefirst contact barrier layer is disposed between the first contact bulklayer and the dielectric layer, and a second portion of the firstcontact barrier layer is disposed between the first via bulk layer andthe dielectric layer. In some embodiments, a first portion of the secondcontact barrier layer is disposed between the second contact bulk layerand the dielectric layer, and a second portion of the second contactbarrier layer is disposed between the third via bulk layer and thedielectric layer.

In some embodiments, the first via having the first via layerconfiguration includes a first via bulk layer, a second via bulk layerdisposed over the first via bulk layer, and a first via barrier layerdisposed between the first via bulk layer and the second via bulk layer.The first via bulk layer physically contacts the dielectric layer andthe first via barrier layer is further disposed between the second viabulk layer and the dielectric layer. In furtherance of such embodiments,the second via having the second via layer configuration includes athird via bulk layer, a fourth via bulk layer disposed over the thirdvia bulk layer, and a second via barrier layer disposed between thethird via bulk layer and the fourth via bulk layer. The third via bulklayer physically contacts the dielectric layer and the second viabarrier layer is further disposed between the fourth via bulk layer andthe dielectric layer. In some embodiments, the first via bulk layer andthe third via bulk layer include a first material, the second via bulklayer and the fourth via bulk layer include a second material, and thefirst via barrier layer and the second via barrier layer include a thirdmaterial. In some embodiments, a first distance between the first viabarrier layer and a substrate is different than a second distancebetween the second via barrier layer and the substrate. In someembodiments, the first source/drain contact includes a first contactbarrier layer and a first contact bulk layer, and the secondsource/drain contact includes a second contact barrier layer and asecond contact bulk layer. A first portion of the first contact barrierlayer is disposed between the first contact bulk layer and thedielectric layer and a second portion of the first contact barrier layeris disposed between the first via bulk layer and the dielectric layer. Afirst portion of the second contact barrier layer is disposed betweenthe second contact bulk layer and the dielectric layer and a secondportion of the second contact barrier layer is disposed between thethird via bulk layer and the dielectric layer.

An exemplary device includes a gate structure disposed over a substrate.The gate structure is disposed between a first source/drain feature anda second source/drain feature. The device further includes a firstsource/drain contact and a second source/drain contact disposed in adielectric layer. The first source/drain contact physically contacts thefirst source/drain feature and the second source/drain contactphysically contacts the second source/drain feature. The device furtherincludes a first via, a second via, and a third via disposed in thedielectric layer. The first via physically contacts the firstsource/drain contact, the second via physically contacts the secondsource/drain contact, and the third via physically contacts the gatestructure. The first via includes a first metal fill layer having firstsidewalls that physically contact the dielectric layer. The second viaincludes a second metal fill layer having second sidewalls thatphysically contact the dielectric layer. The third via includes a thirdmetal fill layer disposed over a metal barrier layer. The metal barrierlayer is disposed between the third metal fill layer and the dielectriclayer, such that third sidewalls of the third metal fill layer do notphysically contact the dielectric layer. In some embodiments, the firstvia and the second via have a first cross-sectional profile along afirst direction and a second cross-sectional profile along a seconddirection. The first direction is different than the second direction. Afirst bottom of the first via and a second bottom of the second via havea substantially planar surface in the first cross-sectional profile anda substantially curved surface in the second cross-sectional profile.

In some embodiments, the metal barrier layer is a first metal barrierlayer. In such embodiments, the first via has a first portion thatincludes the first metal fill layer and a second portion disposed overthe first portion. The second portion includes a fourth metal fill layerdisposed over a second metal barrier layer, the second metal barrierlayer is disposed between the fourth metal fill layer and the dielectriclayer, such that fourth sidewalls of the fourth metal fill layer do notphysically contact the dielectric layer, and the second metal barrierlayer is disposed between the first metal fill layer and the fourthmetal fill layer. In some embodiments, the first via and the second viahave a first thickness, the first metal fill layer has a secondthickness that is less than the first thickness, and the second metalfill layer has a third thickness that is equal to the first thickness.In some embodiments, the second via has a third portion that includesthe second metal fill layer and a fourth portion disposed over the thirdportion. The fourth portion includes a fifth metal fill layer disposedover a third metal barrier layer, the third metal barrier layer isdisposed between the fifth metal fill layer and the dielectric layer,such that fifth sidewalls of the fifth metal fill layer do notphysically contact the dielectric layer, and the third metal barrierlayer is disposed between the second metal fill layer and the fifthmetal fill layer. In some embodiments, the first via and the second viahave a first thickness, the first metal fill layer has a secondthickness that is less than the first thickness, the second metal filllayer has a third thickness that is less than the first thickness, andthe third thickness is different than the second thickness.

In some embodiments, the first source/drain contact includes a firstcontact bulk layer disposed over a first contact barrier layer and thesecond source/drain contact includes a second contact bulk layerdisposed over a second contact barrier layer. A first portion of thefirst sidewalls of the first metal fill layer physically contacts thedielectric layer and a second portion of the first sidewalls of thefirst metal fill layer physically contacts the first contact barrierlayer, such that the first contact barrier layer is disposed between thesecond portion of the first sidewalls of the first metal fill layer andthe dielectric layer. A third portion of the second sidewalls of thesecond metal fill layer physically contacts the dielectric layer and afourth portion of the second sidewalls of the second metal fill layerphysically contacts the second contact barrier layer, such that thesecond contact barrier layer is disposed between the fourth portion ofthe second sidewalls of the second metal fill layer and the dielectriclayer.

An exemplary method includes forming a first source/drain contact and asecond source/drain contact in a dielectric layer. The firstsource/drain contact physically contacts a first source/drain featureand the second source/drain contact physically contacts a secondsource/drain feature. The method further includes forming a first viaopening and a second via opening in the dielectric layer. The first viaopening exposes the first source/drain contact and the second viaopening exposes the second source/drain contact. The method furtherincludes recessing the first source/drain contact to extend the firstvia opening and the second source/drain contact to extend the second viaopening and performing a bottom-up deposition process to form a firstvia bulk layer in the first via opening and a second via bulk layer inthe second via opening. A first thickness of the first via bulk layer isdifferent than a second thickness of the second via bulk layer. Themethod further includes forming a first via barrier layer over the firstvia bulk layer and the second via bulk layer, forming a third via bulklayer over the first via barrier layer, and performing a planarizationprocess to remove any of the third via bulk layer, the first via barrierlayer, the second via bulk layer, and the first via bulk layer that isdisposed over a top surface of the dielectric layer, thereby forming afirst via having a third thickness and a first via layer configurationand a second via having the third thickness and a second via layerconfiguration that is different than the first via layer configuration.The method further includes forming a third via opening in thedielectric layer that exposes a gate structure, forming a second viabarrier layer that partially fills the third via opening, and forming afourth via bulk layer over the second via barrier layer. The fourth viabulk layer fills a remainder of the third via opening. The methodfurther includes performing a planarization process to remove any of thefourth via bulk layer and the second via barrier layer that is disposedover the top surface of the dielectric layer, thereby forming a thirdvia having a third via layer configuration that is different than thefirst via layer configuration and the second via layer configuration. Insome embodiments, forming the first via opening and the second viaopening in the dielectric layer includes performing a dry etchingprocess, and recessing the first source/drain contact and the secondsource/drain contact includes performing a wet etching process. In someembodiments, the first source/drain contact and the second source/draincontact each includes a contact bulk layer disposed over a contactbarrier layer and recessing the first source/drain contact and thesecond source/drain contact includes etching the contact bulk layerwithout substantially etching the contact barrier layer. In someembodiments, each of the forming the third via bulk layer and theforming the fourth via bulk layer includes performing a blanketdeposition process.

Another exemplary method includes forming a source/drain contact to asource/drain feature in a first dielectric layer, forming a first via tothe source/drain contact, and forming a second via to a gate structure.Forming the first via includes forming a second dielectric layer overthe first dielectric layer and forming a first via opening in the seconddielectric layer. The first via opening exposes the source/draincontact. Forming the first via further includes recessing thesource/drain contact to extend the first via opening, performing abottom-up deposition process to fill the extended first via opening witha first metal layer, forming a second metal layer over the first metallayer and the second dielectric layer, forming a third metal layer overthe second metal layer, and performing a planarization process to removeany of the first metal layer, the second metal layer, and the thirdmetal layer disposed over a top surface of the second dielectric layer.Forming the second via includes forming a second via opening in thesecond dielectric layer and the first dielectric layer. The second viaopening exposes the gate structure. Forming the second via furtherincludes forming a fourth metal layer to partially fill the second viaopening and forming a fifth metal layer over the fourth metal layer tofill a remainder of the second via opening. In some embodiments, formingthe first metal layer includes forming a first tungsten-comprising layerand forming the third metal layer includes forming a secondtungsten-comprising layer. In some embodiments, forming the second metallayer includes forming a titanium-comprising layer.

In some embodiments, the extended first via opening has a firstcross-sectional profile along a first direction and a secondcross-sectional profile along a second direction. The first direction isdifferent than the second direction. In such embodiments, a bottom ofthe extended first via opening is defined by a substantially planarsurface in the first cross-sectional profile and a substantiallycurvilinear surface in the second cross-sectional profile. In someembodiments, forming the second metal layer includes depositing a firstglue layer and depositing a second glue layer over the first glue layer.In such embodiments, the first glue layer can include titanium and thesecond glue layer can include titanium and nitrogen. In someembodiments, the first metal layer completely fills the first viaopening, such that the first via includes the first metal layer. In someembodiments, forming the first metal layer includes filling a bottomportion of the first via opening with the first metal layer, such that atop portion of the first via opening remains after the forming the firstmetal layer. In such embodiments, forming the second metal layerincludes partially filling the top portion of the first via opening withthe second metal layer and forming the third metal layer includesfilling a remainder of the top portion of the first via opening with thethird metal layer.

In some embodiments, forming the first via opening includes performing adry etching process and recessing the source/drain contact includesperforming a wet etching process. In some embodiments, the source/draincontact includes a metal fill layer disposed over a metal glue layer andrecessing the source/drain contact includes etching the metal fill layerwithout substantially etching the metal glue layer. In some embodiments,the gate structure has a first dimension along a first direction and asecond dimension along a second direction that is substantiallyperpendicular to the first direction. In such embodiments, recessing thesource/drain contact to extend the first via opening extends the firstvia opening under the second dielectric layer along the second directionwithout extending the first via opening under the second dielectriclayer along the first direction. In some embodiments, the firstdimension is a gate length and the second dimension is a gate width.

In some embodiments, the source/drain contact is a first source/draincontact and the source/drain feature is a first source/drain feature. Insuch embodiments, the method can further include forming a secondsource/drain contact to a second source/drain feature in the firstdielectric layer. The gate structure is disposed between the firstsource/drain feature and the second source/drain feature. In suchembodiments, the method can further includes forming a third via in thesecond dielectric layer to the second source/drain contact while formingthe first via to the first source/drain contact. Forming the third viaincludes forming a third via opening in the second dielectric layer. Thethird via opening exposes the second source/drain contact. Forming thethird via further includes recessing the second source/drain contact toextend the third via opening, performing the bottom-up depositionprocess to fill the extended third via opening with the first metallayer, forming the second metal layer over the first metal layer and thesecond dielectric layer, forming the third metal layer over the secondmetal layer, and performing the planarization process to remove any ofthe first metal layer, the second metal layer, and the third metal layerdisposed over the top surface of the second dielectric layer. In someembodiments, the first metal layer partially fills the first via openingand completely fills the third via opening and extends over the seconddielectric layer, thereby forming a first portion of the first metallayer disposed in the first via opening and a second portion of thefirst metal layer disposed in the third via opening. In someembodiments, the second metal layer partially fills the first viaopening and is disposed over the first portion of the first metal layerdisposed in the first via opening and the second metal layer is disposedover the second portion of the first metal layer disposed in the thirdvia opening. In some embodiments, the third metal layer partially fillsa remainder of the first via opening and is disposed over the secondmetal layer.

Another exemplary device includes a gate structure disposed over asubstrate. The gate structure is disposed between a first source/drainfeature and a second source/drain feature. The device further includes afirst source/drain contact and a second source/drain contact disposed ina dielectric layer. The first source/drain contact physically contactsthe first source/drain feature and the second source/drain contactphysically contacts the second source/drain feature. The device furtherincludes a first via, a second via, and a third via disposed in thedielectric layer. The first via physically contacts the firstsource/drain contact, the second via physically contacts the secondsource/drain contact, and the third via physically contacts the gatestructure. The first via and the second via each include a first metalfill layer that physically contacts the dielectric layer. The third viahas a second metal fill layer and a metal glue layer. The metal gluelayer is disposed between the second metal fill layer and the dielectriclayer. In some embodiments, the metal glue layer is a first metal gluelayer, the first via has a first portion where the first metal filllayer physically contacts the dielectric layer, and the first via has asecond portion where a second metal glue layer is disposed between thefirst metal fill layer and the dielectric layer. In some embodiments,the first source/drain contact and the second source/drain contact eachinclude a third metal glue layer disposed between a third metal filllayer and the dielectric layer. In such embodiments, the first viafurther includes a third portion where the third metal glue layer isdisposed between the first metal fill layer and the dielectric layer andthe second via includes a portion where the third metal glue layer isdisposed between the first metal fill layer and the dielectric layer. Insome embodiments, the first via and the second via have a firstcross-sectional profile along a first direction and a secondcross-sectional profile along a second direction. The first direction isdifferent than the second direction. In such embodiments, a bottom ofthe first via and a bottom of the second via are defined by asubstantially planar surface in the first cross-sectional profile and asubstantially curvilinear surface in the second cross-sectional profile.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A device comprising: a first source/drain contactdisposed in a dielectric layer, wherein the first source/drain contactphysically contacts a first source/drain feature; a second source/draincontact disposed in the dielectric layer, wherein the secondsource/drain contact physically contacts a second source/drain feature;a first via having a first via layer configuration disposed in thedielectric layer, wherein the first via extends into and physicallycontacts the first source/drain contact and the first source/draincontact physically contacts sidewalls of the first via; and a second viahaving a second via layer configuration disposed in the dielectriclayer, wherein the second via extends into and physically contacts thesecond source/drain contact, the second source/drain contact physicallycontacts sidewalls of the second via, the second via layer configurationis different than the first via layer configuration, and a firstthickness of the first via is the same as a second thickness of thesecond via.
 2. The device of claim 1, wherein: the first via includes afirst upper portion disposed over a first lower portion, wherein thedielectric layer physically contacts sidewalls of the first upperportion and the first source/drain contact physically contacts sidewallsof the first lower portion; and the second via includes a second upperportion disposed over a second lower portion, wherein the dielectriclayer physically contacts sidewalls of the second upper portion and thesecond source/drain contact physically contacts sidewalls of the secondlower portion.
 3. The device of claim 1, wherein each of the first viaand the second via have a first cross-sectional profile along a firstdirection and a second cross-sectional profile along a second direction,wherein the first direction is different than the second direction andthe first cross-sectional profile is different than the secondcross-sectional profile.
 4. The device of claim 1, wherein: each of thefirst via and the second via have a first bottom interface profile withthe first source/drain contact and the second source/drain contact,respectively, along a first direction and a second bottom interfaceprofile with the first source/drain contact and the second source/draincontact, respectively, along a second direction; and the first directionis different than the second direction, the first bottom interfaceprofile is defined by a substantially planar surface, and the secondbottom interface profile is defined by a substantially curved surface.5. The device of claim 1, wherein: the dielectric layer includes a firstinterlevel dielectric layer, a second interlevel dielectric layerdisposed over the first interlevel dielectric layer, and a contact etchstop layer disposed between the first interlevel dielectric layer andthe second interlevel dielectric layer; the first source/drain contactand the second source/drain contact are disposed in and extend throughthe first interlevel dielectric layer; and the first via and the secondvia are disposed in and extend through the second interlevel dielectriclayer and the contact etch stop layer, wherein a first portion of thefirst via extending into the first source/drain contact physicallycontacts a bottom surface of the contact etch stop layer and a secondportion of the second via extending into the second source/drain contactphysically contacts the bottom surface of the contact etch stop layer.6. The device of claim 1, wherein: the first via having the first vialayer configuration includes: a first via bulk layer, a second via bulklayer disposed over the first via bulk layer, and a via barrier layerdisposed between the first via bulk layer and the second via bulk layer,wherein the first via bulk layer physically contacts the dielectriclayer and the via barrier layer is further disposed between the secondvia bulk layer and the dielectric layer; and the second via having thesecond via layer configuration includes: a third via bulk layer thatphysically contacts the dielectric layer, wherein the first via bulklayer and the third via bulk layer include a same material, the thirdvia bulk layer has the second thickness, and the first via bulk layerhas a third thickness that is less than the first thickness.
 7. Thedevice of claim 6, wherein: the first source/drain contact includes afirst contact barrier layer and a first contact bulk layer, wherein afirst portion of the first contact barrier layer is disposed between thefirst contact bulk layer and the dielectric layer and a second portionof the first contact barrier layer is disposed between the first viabulk layer and the dielectric layer; and the second source/drain contactincludes a second contact barrier layer and a second contact bulk layer,wherein a first portion of the second contact barrier layer is disposedbetween the second contact bulk layer and the dielectric layer and asecond portion of the second contact barrier layer is disposed betweenthe third via bulk layer and the dielectric layer.
 8. The device ofclaim 1, wherein: the first via having the first via layer configurationincludes: a first via bulk layer, a second via bulk layer disposed overthe first via bulk layer, and a first via barrier layer disposed betweenthe first via bulk layer and the second via bulk layer, wherein thefirst via bulk layer physically contacts the dielectric layer and thefirst via barrier layer is further disposed between the second via bulklayer and the dielectric layer; the second via having the second vialayer configuration includes: a third via bulk layer, a fourth via bulklayer disposed over the third via bulk layer, and a second via barrierlayer disposed between the third via bulk layer and the fourth via bulklayer, wherein the third via bulk layer physically contacts thedielectric layer and the second via barrier layer is further disposedbetween the fourth via bulk layer and the dielectric layer; wherein thefirst via bulk layer and the third via bulk layer include a firstmaterial, the second via bulk layer and the fourth via bulk layerinclude a second material, and the first via barrier layer and thesecond via barrier layer include a third material; and wherein a firstdistance between the first via barrier layer and a substrate isdifferent than a second distance between the second via barrier layerand the substrate.
 9. The device of claim 8, wherein: the firstsource/drain contact includes a first contact barrier layer and a firstcontact bulk layer, wherein a first portion of the first contact barrierlayer is disposed between the first contact bulk layer and thedielectric layer and a second portion of the first contact barrier layeris disposed between the first via bulk layer and the dielectric layer;and the second source/drain contact includes a second contact barrierlayer and a second contact bulk layer, wherein a first portion of thesecond contact barrier layer is disposed between the second contact bulklayer and the dielectric layer and a second portion of the secondcontact barrier layer is disposed between the third via bulk layer andthe dielectric layer.
 10. A device comprising: a gate structure disposedover a substrate, wherein the gate structure is disposed between a firstsource/drain feature and a second source/drain feature; a firstsource/drain contact and a second source/drain contact disposed in adielectric layer, wherein the first source/drain contact physicallycontacts the first source/drain feature and the second source/draincontact physically contacts the second source/drain feature; and a firstvia, a second via, and a third via disposed in the dielectric layer,wherein: the first via physically contacts the first source/draincontact, the second via physically contacts the second source/draincontact, and the third via physically contacts the gate structure, thefirst via includes a first metal fill layer having first sidewalls thatphysically contact the dielectric layer, the second via includes asecond metal fill layer having second sidewalls that physically contactthe dielectric layer, and the third via includes a third metal filllayer disposed over a metal barrier layer, wherein the metal barrierlayer is disposed between the third metal fill layer and the dielectriclayer, such that third sidewalls of the third metal fill layer do notphysically contact the dielectric layer.
 11. The device of claim 10,wherein: the metal barrier layer is a first metal barrier layer; and thefirst via has a first portion that includes the first metal fill layerand a second portion disposed over the first portion, wherein the secondportion includes a fourth metal fill layer disposed over a second metalbarrier layer, the second metal barrier layer is disposed between thefourth metal fill layer and the dielectric layer, such that fourthsidewalls of the fourth metal fill layer do not physically contact thedielectric layer, and the second metal barrier layer is disposed betweenthe first metal fill layer and the fourth metal fill layer.
 12. Thedevice of claim 11, wherein the first via and the second via have afirst thickness, the first metal fill layer has a second thickness thatis less than the first thickness, and the second metal fill layer has athird thickness that is equal to the first thickness.
 13. The device ofclaim 11, wherein: the second via has a third portion that includes thesecond metal fill layer and a fourth portion disposed over the thirdportion, wherein the fourth portion includes a fifth metal fill layerdisposed over a third metal barrier layer, the third metal barrier layeris disposed between the fifth metal fill layer and the dielectric layer,such that fifth sidewalls of the fifth metal fill layer do notphysically contact the dielectric layer, and the third metal barrierlayer is disposed between the second metal fill layer and the fifthmetal fill layer.
 14. The device of claim 13, wherein the first via andthe second via have a first thickness, the first metal fill layer has asecond thickness that is less than the first thickness, the second metalfill layer has a third thickness that is less than the first thickness,and the third thickness is different than the second thickness.
 15. Thedevice of claim 10, wherein: the first via and the second via have afirst cross-sectional profile along a first direction and a secondcross-sectional profile along a second direction, wherein the firstdirection is different than the second direction; and a first bottom ofthe first via and a second bottom of the second via have a substantiallyplanar surface in the first cross-sectional profile and a substantiallycurved surface in the second cross-sectional profile.
 16. The device ofclaim 10, wherein: the first source/drain contact includes a firstcontact bulk layer disposed over a first contact barrier layer and thesecond source/drain contact includes a second contact bulk layerdisposed over a second contact barrier layer; a first portion of thefirst sidewalls of the first metal fill layer physically contacts thedielectric layer and a second portion of the first sidewalls of thefirst metal fill layer physically contacts the first contact barrierlayer, such that the first contact barrier layer is disposed between thesecond portion of the first sidewalls of the first metal fill layer andthe dielectric layer; and a third portion of the second sidewalls of thesecond metal fill layer physically contacts the dielectric layer and afourth portion of the second sidewalls of the second metal fill layerphysically contacts the second contact barrier layer, such that thesecond contact barrier layer is disposed between the fourth portion ofthe second sidewalls of the second metal fill layer and the dielectriclayer.
 17. A semiconductor structure, comprising: a first dielectriclayer, a second dielectric layer disposed over the first dielectriclayer, and a third dielectric layer disposed over the second dielectriclayer; a first interconnect structure that includes a first metal plug,a second metal plug, and a first barrier layer, wherein the firstbarrier layer is disposed between the first metal plug and the firstdielectric layer, the first barrier layer is disposed between the firstmetal plug and the second dielectric layer, the first barrier layer isdisposed between the second metal plug and the second dielectric layer,the second metal plug is disposed over and physically contacts the firstmetal plug, and the second metal plug physically contacts the thirddielectric layer; a second interconnect structure that includes a thirdmetal plug, a fourth metal plug, and a second barrier layer, wherein thesecond barrier layer is disposed between the third metal plug and thefirst dielectric layer, the second barrier layer is disposed between thethird metal plug and the second dielectric layer, the second barrierlayer is disposed between the fourth metal plug and the seconddielectric layer, the fourth metal plug is disposed over and physicallycontacts the third metal plug, and the fourth metal plug physicallycontacts the third dielectric layer; and wherein a first thickness ofthe second metal plug is different than a second thickness of the fourthmetal plug.
 18. The semiconductor structure of claim 17, wherein thesecond interconnect structure further includes a fifth metal plug and athird barrier layer, wherein the fifth metal plug is disposed over thefourth metal plug, the third barrier layer is disposed between the fifthmetal plug and the third dielectric layer, and the third barrier layeris disposed between the fifth metal plug and the fourth metal plug. 19.The semiconductor structure of claim 17, wherein: a first portion of thesecond metal plug is disposed in the second dielectric layer and asecond portion of the second metal plug is disposed in the thirddielectric layer; a first portion of the fourth metal plug is disposedin the second dielectric layer and a second portion of the fourth metalplug is disposed in the third dielectric layer; the first portion of thesecond metal plug has a third thickness, the second portion of thesecond metal plug has a fourth thickness, the first portion of thefourth metal plug has a fifth thickness, and the second portion of thefourth metal plug has a sixth thickness; the first thickness is a sum ofthe third thickness and the fourth thickness and the second thickness isa sum of the fifth thickness and the sixth thickness; and the thirdthickness is about equal to the fifth thickness and the fourth thicknessis different than the sixth thickness.
 20. The semiconductor structureof claim 17, wherein: the second metal plug has a first width in thesecond dielectric layer and a second width in the third dielectriclayer, wherein the first width is less than the second width; and thesecond metal plug has a third width in the second dielectric layer and afourth width in the third dielectric layer, wherein the third width isless than the fourth width.